Math 640: EXPERIMENTAL MATHEMATICS
Spring 2008 (Rutgers University) Webpage
http://sites.math.rutgers.edu/~zeilberg/math640_08.html
Last Update: Oct. 11, 2008.
Added Oct. 11, 2008:
Here are the awsome
final projects done by the students
 Teacher:
Dr. Doron ZEILBERGER ("Dr. Z")

Classroom:
Allison Road Classroom Building
[Busch Campus], IML Room 119 [Inside computer lab]

Time: Mondays and Thursdays , period 3 (12:00noon1:20pm)

"Textbook": handouts.

Dr. Zeilberger's Office: Hill Center 704 ( Phone: (732) 4451326)

Dr. Zeilberger's Email:
zeilberg at math dot rutgers dot edu

Dr. Zeilberger's Office Hours: MTh 10:0010:01 and by appointment
Description
Experimental Mathematics used to be considered an oxymoron, but the future of mathematics is in this direction.
In addition to learning the philosophy and methodology of this budding field, students will become computeralgebra
wizards, and that should be very helpful in whatever mathematical specialty they'll decide to do research in.
We will first learn Maple, and how to program in it. This semester we will explore
Automated (symbolic!) Enumeration, that consists of teaching the computer how to
find explicit formulas, and/or general algorithms, for enumerating combinatorial objects.
But the actual content is not that important, it is mastering the methodology
of computergenerated and computerassisted research that is so crucial for your future.
There are no prerequisites, and no previous programming knowledge is assumed. Also, very little overlap
with previous years. The final projects for this class may lead to journal publications.
Here are the suggested
final projects .
Diary and Homework
Programs done on Thurs., Jan. 24, 2008

jan24.txt ,
a program that inputs an integer p and outputs true or false
according to whether or not a^{p} mod p = a for all a between
1 and p1.
Homework for Thurs., Jan. 24, class (due Jan. 28, 2008)

Read and do all the examples, plus make up similar ones,
in the first 30 pages of Frank Garvan's
awesome Maple booklet.

Write a Maple program that inputs positive integers a and K
and prints out all the integers n, less than K that are
not prime, yet satisfy a^{n} mod n = a .
Programs done on Mon., Jan. 28, 2008

jan28.txt
,
contains the program choose1(S,k) that inputs a set S
and a nonnegative integer k, and outputs the set of
subsets of S with exactly k elements, and Fnk(n,k) the
inputs nonnegative integers n and k and outputs the
set of lists of length k, whose entries are 1 or 2, and
that add up to n.
Homework for Mon., Jan. 28, class (due Thurs., Jan. 31, 2008)
 For Newcomers:

Read and do all the examples, plus make up similar ones,
in pages 3060 of Frank Garvan's
awesome Maple booklet. Print out a few sample examples, that you
made up on your own, and hand them in.

Modify Fnk(n,k) to write a Maple program, call it Gnk(n,k) that
inputs nonnegative integers n and k and outputs the
set of lists of length k, whose entries are from the set {1,2,3}.

Using Fnk(n,k) as a subroutine
write a program Fn(n) that inputs a nonnegative integer n, and outputs
the set of lists whose entries are drawn from {1,2} any length,
that sumop to n.

Without using Fnk(n,k) as a subroutine,
write a program Fn1(n) that inputs a nonnegative integer n, and outputs
the set of lists whose entries are drawn from {1,2} any length,
that sumop to n.

What is the sequence nops(Fn(n)), n=1,2,3, ... ?
 For Oldtimers: (updated Jan. 29, 2008, correcting errors pointed
out by Lara Pudwell)

Make sure that your mentee
read and does all the examples, plus make up similar ones,
in pages 3060 of Frank Garvan's
awesome Maple booklet. Also, make sure that they can do
the homework.
If he or she have any problems, help
them, but don't do it for them.

Modify Fnk(n,k) to write a Maple program, call it FSnk(S,n,k) that
inputs a set of integers, S, and nonnegative integers n and k
and outputs the
set of lists of length k, whose entries are from the set S,
and that sum to n.

Using FSnk(S,n,k) as a subroutine
write a program FSn(S,n) that inputs a set of positive
integers and a nonnegative integer n, and outputs
the set of lists whose entries are drawn from the set S, of any length,
that sumop to n.

Without using FSnk(S,n,k) as a subroutine,
write a program FSn1(S,n) that inputs
and a set of positive integers S, and a nonnegative integer n, and outputs
the set of lists whose entries are drawn from the set S, of any length,
that sumop to n.

What is the (ordinary) generating function for the
sequence nops(FSn(S,n)), n=0,1,2,3, ... ?
Programs done on Thurs., Jan. 31, 2008

jan31.txt
,
in addition to the previous programs in jan28.txt, contains
programs for computing the powerset of a set S, as well
as Fn(n), that outputs the set of sequences of 1's and 2's
that adds up to n.
Homework for Thurs. Jan. 31, class (due Mon., Feb. 4, 2008)
 For Newcomers:

Read and do all the examples, plus make up similar ones,
in pages 6190 of Frank Garvan's
awesome Maple booklet. Print out a few sample examples, that you
made up on your own, and hand them in.

Write a program Seq01(n), that inputs a nonnegative integer n,
and outputs the set of
all sequences of length n, whose entries are drawn from {0,1}.
For example, Seq01(2); should yield {[0,0],[0,1],[1,0],[1,1]};

Using Seq01(n), write a program, G01(n),
that inputs a nonnegative integer n,
and outputs the subset of Seq01(n) of those sequences
that never have two 0's in a row.
For example, G01(3); should give
{[0,1,0],[1,0,1],[0,1,1],[1,1,0],[1,1,1]}.
 For OldTimers:

Make sure that your mentee does all the examples,
and completely masters pages 6190 of Frank Garvan's
awesome Maple booklet.

Write a program SeqS(S,n), that inputs a set S and a nonnegative integer n,
and outputs all sequences of length n whose entries are drawn from S.
For example, SeqS({0,1},2); should yield {[0,0],[0,1],[1,0],[1,1]};

Using SeqS(S,n), write a program SeqG(S,T,n)
that inputs a set of integers S, a set of lists T (in the alphabet S),
and a nonnegative integer n,
and outputs the subset of SeqS({0,1},n) of those sequences
that do not have any "factors" (i.e. contiguous subwords) in T.
For example, SeqG({0,1},{[0,0]}, 3); should give
{[0,1,0],[1,0,1],[0,1,1],[1,1,0],[1,1,1]}.
Programs done on Feb. 4

feb4.txt
,
contains bruteforce programs for computing the set of sequences
whose entries are from S or their negatives, and such that all partial
sums (except the 0th) are never zero.
Homework for Feb. 4 class (due Feb. 7)
 For Newcomers:

Read and do all the examples, plus make up similar ones,
in pages 91120 of Frank Garvan's
awesome Maple booklet. Print out a few sample examples, that you
made up on your own, and hand them in.

I just realized that it was stupid to have the input consist of
a set S of positive integers, and then invite their negatives in.
Modify TieLessGamesC(S,n), so that the input is any set of
numbers S (do not have to be integers), and a nonnegative integer,
n, and outputs the set of lists of length n, whose elements are drawn
from S, such that no partial sum (except the 0th, of course) is ever zero.

Using the new TieLessGamesC(S,n) that you have just written,
modify it to write a program fSTn(S,T,n), that inputs two sets
of numbers, S, and T, and a nonnegative integer n, and outputs
the set of lists of length n, whose entries are drawn from the set S,
and such that no partial sum (except possibly the 0th) belongs to T.
 For OldTimers:

Make sure that your mentee
reads and does all the examples, and makes up similar ones,
in pages 91120 of Frank Garvan's
awesome Maple booklet.

I just realized that it was stupid to have the input consist of
a set S of positive integers, and then invite their negatives in.
Modify TieLessGamesC(S,n), so that the input is any set of
numbers S (that do not have to be integers), and a nonnegative integer,
n, and outputs the set of lists of length n, whose elements are drawn
from S, such that no partial sum (except the 0th, of course) is ever zero.

(Version of Feb. 5, 2008, correcting the previous
version that it didn't make sense, thanks to Emilie Hogan!)
Write a slightly more general program then the above TieLessGamesC(S,n),
call it GenTieLessGamesC(S,n,k), that inputs a set of numbers S,
a number k, and a nonnegative integer n, and outputs the set of
lists of length n, whose entries are drawn from S, none of whose
partial sums are 0, and in addition the (full) sum of their
entries equals k. Note that
TieLessGamesC(S,n)=Union(GenTieLessGamesC(S,n,k); k from n*min(S) to n*max(S), and k NOT 0)

By mimicking the above program, write an enumeration program, let's call
it fSkn(S,n,k), that finds the number of elements of
GenTieLessGamesC(S,n,k). Compare the output to nops(GenTieLessGamesC(S,n,k)),
for small n and k.

For the following games, how many tieless games, with 50 scoring "events"
are there
 Soccer (S={1,1})
 OldTime Basketball (S={1,2,1,2})
 Contemporary Basketball (S={1,2,3,1,2,3})
 [American] Football (S=?, I forgot, look it up, or ask Lara)
(If it runs for too long, replace 50 by a smaller number)
Programs done on Feb. 7 class

feb7.txt
, contains fSn(S,n): a program that inputs a set of integers
S and outputs the number of nletter words in the alphabet S,
none of whose partial sums (except the 0th, of course) are 0,
and fSnSeq(S,N) that gives the first N terms of this sequence.

FindRec.txt ,
a program that guesses recurrences and analyzes sequences,
do ezra(); to find out.
Homework for the Feb. 8, class (due Feb. 11, 2008)
 For Newcomers

Read and do all the examples, plus make up similar ones,
from page 121 all the way to the end, of Frank Garvan's
awesome Maple booklet. Print out a few sample examples, that you
made up on your own, and hand them in.

Modify fSn(S,n) to write a program
fSTn(S,T,n) that inputs two sets of integers S and T, and
an integer n, and outputs the number of nletter words
in the alphabet S, none of whose nonempty partial sums has
an element of T. Note that fSTn(S,{0},n) equals fSn(S,n) .

Using Findrec(L,n,N,12); in
FindRec.txt ,
find linear recurrences for various S's and T's.

I roll a standard die 100 times. If it shows 1,2,3, I lose 3,2,1 dollars respectively.
If it shows 4,5,6, I win 1,2,3, dollars respectively. What is the exact probability
that I never had in my possession neither 2 dollars, nor 5 dollars, nor, 3 dollars?
 For OldTimers:

Make sure that your mentee
reads and does all the examples, and makes up similar ones,
pages 120 all the way to the end, of Frank Garvan's
awesome Maple booklet.

Write a program, WSTn(S,T,n) that inputs a set of steps in 2 dimensions given as
lists of length 2, (for example, for the simple random walk it is
{[1,0],[1,0],[0,1],[0,1]}), as well as a set
of lattice points T, and an integer n, and outputs the number of
nstep walks, with steps drawn from the set S, that never ever visit
any of the points of T.
Programs done on Feb 11 class

feb11.txt , An algebraic approach to finding the
number of sequences of length n in an alphabet of integers S, none
of whose (nontrivial) partial sums belong to a set of integers T.

WalkPlot, Andrew Baxter's neat program that plots
both 2D and 3D walks.
Homework for Feb 11 class (due Feb 14)
 For Newcomers:

A generous beggar, who also donates to other beggars,
receives or gives away, one coin at a time, that could be
either a penny, a nickel, a dime, or a quarter.
If there were 1000 events, what is the exact probability that,
he was never in the red
(in other words, he never had 1, 2, ..., 25 cents) .?

Let a(n) be the number of
oldtime basketball games with n scoring events that ended in a tie,
and where the home team was never behind. Find, empirically,
a recurrence for a(n).

What is the probability of a football games with 20 scoring events
ending with a tie? (assume, unrealistically, that each scoring event
is equally likely).

Write a program that inputs an integer r, and outputs the
numbers a_{r} and b_{r} such that
Σ_{ 0 ≤ k ≤ n} binomial(n,k)^{r}
is asymptotic to (a_{r})^{n} n^{br} .
Can you conjecture what a_{r} and b_{r} are in general
(as expressions in r?)

Write a program that inputs a positive integer n, and outputs the
number of integer partitions of n. For example p(3)=3, since
3 can be written as 1+1+1, 2+1, 3. Do you notice anything about
p(5n+4) ?
 For OldTimers:

A walk is selfavoiding if it never visits the same place twice.
Write a program that inputs a set of integers S, and a nonnegative
integer n, and outputs the number of onedimensional
selfavoiding walks of length n, using steps from S.

Generalize the above for twodimensional walks.
Find the first 10 terms (or whatever you can) terms in
the sequence of 2D simple selfavoiding walks, i.e.,
where S={[1,0],[1,0],[0,1],[0,1]};
Programs done on Feb. 14, 2008, class
Homework for Feb 14 class (due Feb. 21)
Programs done on Feb 18, 2008
(Guest Lecturer: Eric Rowland,
made a comparative study of Maple vs. Mathematica)
feb18.nb
Homework (To be handedin to Eric, due Feb. 25, 2008)

Take a simple function that you've written recently, and rewrite it
(in your language of choice) using functional (rather than procedural)
constructs.

At 3am early Monday morning you discover that the Riemann hypothesis
reduces to the statement that all trianglefree Hamiltonconnected graphs
with 7 vertices satisfy a certain simple condition. Use GraphData[ ] to
find all graphs that you need to check. Will you be able to get eight
hours of sleep and still make it to Experimental Math on time?

Make something cool with Manipulate[ ], and if it's sufficiently cool
then upload it to the Demonstrations Project.
Program done in Feb. 21, 2008 class
feb21.txt
(Under construction, to be completed next time)
Homework for Feb 21, 2008 (due Feb. 25, 2008)

For Newcomers

Write a program IsFactor(u,w) that inputs two words (given as lists) and
outputs true if and only if u is a factor of w
(which means that there exist i < j such that w[i]w[i+1]...w[j]=u

Write a program AllWords(A,n) that inputs an alphabet A, and a nonneg. integer,
and outputs all A^{n} nletter words (lists) in the alphabet A.

Write a program, GoodWords(A,F,n), that inputs a set A, a set F of lists in A, and
a nonneg. integer n, and outputs all the nletter words in A, that do not contain
(as a factor) any of the words of F.

using GoodWords(A,F,n), and nops, write a program, SeqGood(A,F,n), that inputs A,F,n and outputs
a list of integers, of length n, whose i^{th} term is the number of nletter
words in the alphabet A that do not contain, as factors, any of thw words of F.

For Oldtimers: All the above for newcomers, PLUS
Complete, on your own,
feb21.txt ,
and compare notes with SeqGood(A,F,n) .
Program done on Feb. 25, 2008 class
Recommended Reading:
Enumerating Totally Clean Words by Dr. Z., that outlines
the algorithm for counting words that omit (as subwords) a
given set of "dirty" words.
feb25.txt,
contains

the function fAFt(A,F,t), that inputs a set of letters
("alphabet"), and a set of "dirty words", F, (using the letters of A),
and a variable t, and outputs the rational function in t, that is
the generating function for the sequence "number of nletter words
in the alphabet A, not containing as "factors", any of the words of F."

the function , SeqC(A,F,N), that inputs a set of letters
("alphabet"), and a set of "dirty words", F, (using the letters of A),
and a positive integer N, and outputs the list of length N,
whose n^{th} term is
the number of nletter words
in the alphabet A, not containing as "factors", any of the words of F.
Homework for Feb. 25, 2008 (due Feb. 28, 2008)

For Newcomers:

If you toss a fair coin 100 times, what is the probability
that you have neither 6 Headsinarow nor 6 Taillsinarow?

What is the generating function for the number of
nletter words in the alphabet {1,2,3,4} that never
contain four consecutive letters that are all different?
Calling the counting sequence a(n), find constants C and μ
such that a(n) is asymptotic to Cμ^{n}.

Adapt fAFt(A,F,t) to write a program fAFtx(A,F,t,x) that has the
same input as fAFt(A,F,t) and in addition another symbol x, and
outputs the rational function in (t,x[A[1]],x[A[2]], ... ) whose
Maclaurin expansion in t has for its coefficient of t^{n}
the POLYNOMIAL in (x[A[1]],x[A[2]], ...) that is the weightenumerator
of the set of nletter words in A, avoiding F, where the
weight of a word w is x[w[1]]x[w[2]]...
(for example, the weight of ESSEX is x[E]x[S]x[S]x[E]x[X]=
x[E]^{2}x[S]^{2}x[X]).
For example, fAFtx({1,2},{},t,x); should be
1/(1t*(x[1]+x[2]));

Read carefully pages 19 in the highly readable and entertaining
article, by Dr. Z. and John Noonan.

For Oldtimers: ALL the above plus:

Write a new version of fAFt(A,F,t), call it, gAFt(A,F,t),
implementing the GouldenJackson method. Do not peek
at any Maple packages that may be available online. Test that
your gAFt(A,F,t) yields the same output as fAFt(A,F,t), for
various randomlychosen A and F.
Program done on Feb. 28, 2008, class
feb28.txt,
contains

the function gAFt(A,F,t), that does exactly what
fAFt(A,F,t) does in
feb25.txt,
but using the GouldenJackson cluster approach.
Homework for Feb. 28, 2008 (due March 3, 2008)
For everyone
(Note: from now on everybody has the same homework, but I will understand
if newcomers won't do everything).

Adapt gAFt(A,F,t) to write a program gAFtx(A,F,t,x) that has the
same input as gAFt(A,F,t) and in addition another symbol x, and
outputs the rational function in (t,x[A[1]],x[A[2]], ... ) whose
Maclaurin expansion in t has for its coefficient of t^{n}
the POLYNOMIAL in (x[A[1]],x[A[2]], ...) that is the weightenumerator
of the set of nletter words in A, avoiding F, where the
weight of a word w is x[w[1]]x[w[2]]...
(for example, the weight of ESSEX is x[E]x[S]x[S]x[E]x[X]=
x[E]^{2}x[S]^{2}x[X]).
For example, gAFtx({1,2},{},t,x); should be
1/(1t*(x[1]+x[2]));

Adapt gAFt(A,F,t) to write a program gAFts(A,F,t,s) that has the
same input as gAFt(A,F,t) and in addition another symbol s, and
outputs the rational function in (t,s ) whose
Maclaurin expansion in (t,s) has for its coefficient of t^{n}s^{m}
the number of nletter words in the alphabet A, that have exactly m mistakes.
Note that gAFts(A,F,t,0) should equal gAFt(A,F,t).
For example, gAFts({1,2},{[1,2]},t,s); should be
1/(12*t(s1)*t^2);
Hint: replace 1 by (s1) in gAFt(A,F,t) (and/or its subroutines)

Write a test program Test(r,m,k), that inputs integers r and m and k, that verifies that
fAFt and gAFt yield the same output,
for A={1,2,...,r}, and for EVERY possible kelement set of mlettered "dirty words"
and see who is faster
For A={1,...,r} each such F
t0:=time(): read `feb25.txt`: a:=fAFt(A,F,t); time()t0;
t0:=time(): read `feb28.txt`: b:=gAFt(A,F,t); time()t0;
evalb(a=b);
Actually run Test(2,3,3); and if you can Test(2,3,4);

Adapt gAFt(A,F,t), to write a program
gPFt(P,F,t), that inputs a probability vector P (whose length is the length of the
alphabet) and where P[i] is the prob. that the i^{th} letter shows up,
and outputs the rational function whose Maclaurin expansion, yields, as
coeff. of t^{n}, the prob. that a random nletter word will not
contain any occurrence of the "dirty words" of F.

Look up the frequencies of English letters, and determine the probability
that a 200letter random "word" in English is SEXless.

(5 dollar reward for the champion).
Find an example of A and F where gAFt(A,F,t) beats, in running time
fAFt(A,F,t) by as big a factor as possible (or vice versa).
Programs done on March 3, 2008, class
mar3.txt,
contains

G(n): a program that inputs a pos. integer n, and outputs
the set of simple labelled graphs on {1, ..., n}

Cc(G,v): a program that inputs a graph G, and a vertex v,
and outputs the set of vertices that are connected, via
some path, to v.

IsConnected(G,n): inputs a graph G and a pos. integer n, and
outputs true iff the graph G (on the set of vertices {1, ..., n})
is connected.

CG(n): inputs a pos. integer n, and outputs the set of
labelled connected graphs on {1, ..., n} .
Homework for March 3, 2008 (due March 6, 2008)

Print out, and carefully read
Dr. Z.'s crash course on enumerative combinatorics

Modify G(n) to write a program
Gnk(n,k), that inputs pos. integers n and k, and outputs
the set of simple labelled graphs on {1, ..., n} with exactly
k edges.

Modify CG(n) to write a program
CGnk(n,k), that inputs pos. integers n and k, and outputs
the set of simple connected labelled graphs on {1, ..., n} with exactly
k edges.

Compute [seq(nops(CGnk(n,n1)),n=1..7)], and conjecture an explicit
expression for the number of connected labelled graphs with n vertices
and n1 edges.

Write a program, called Image(G,n,pi) that inputs a labelled grap G on
the set of vertices {1, ..., n} and a permutation pi on {1, ..., n}
and outputs the graph on {1, ..., n} obtained by renaming i by pi[i]
(for all i=1...n).

Using permute(n) of the combinat package, write a program
Images(G,n), that inputs a labelled graph G on {1, ..., n}
and outputs the set of all images of G under permutations of {1, ..., n}

An unlabeled graph on n vertices may be viewed as an
equivalence class of labelled graphs on {1, ..., n} under
the equivalence relation "being an image under a permutation".
So you can represent an unlabeled graph as
a set of labelled graphs. For example the equivalence class
of
{[1,2],[1,3}} is
{{[1,2],[1,3}}, {[1,2],[2,3}}, {[1,3],[2,3}}} .
Write a program ULCGnk(n,k), the inputs pos. integers n and k, and outputs
all unlabeled connected graphs on n vertices and k edges.

Compute
[seq(nops(ULGCnk(n,n1)),n=1..7)]:
and search for it in Sloane.
Congratulations to
Eric Rowland
for winning the
$10 prize for the longest GouldenJacksonstyle
cluster
Programs done on March 6, 2008, class

mar6.txt,
(finishing up mar3.txt, adding procedures for rapid counting of
connected labelled graph and connected labelled graph with
a given number of edges, and the "stupid" way to count
unlabeled connected graphs.

mar6a.txt,
direct counting of labelled trees (under construction, to be continued by you!,
see homework).
Homework for March 6, 2008 (due March 10, 2008)

Modify UCG(n) of
mar6.txt,
to write a program UCGe(n,e) that inputs pos. integers n and e, and
outputs the "set" of unlabeled graphs on n vertices and n1+e edges.
For example, UCGe(n,0) should give the set of unlabeled trees on n vertices
(or rather one representative labelled tree from each equivalence class)

Modify NuUCGs(n) in
mar6.txt,
to write a program NuUCGse(n,e) that inputs pos. integers n and e, and
outputs the counting sequence, from i=1 to i=n for the number
of unlabeled trees on n vertices and n1+e edges.
For example, NuUCGse(n,0) should give the sequence for the number of unlabeled trees on i vertices
for i from 1 to n .
Look up NuCGse(6,0); in Sloane

Finish up RT(S) of
mar6a.txt,
as follows.

Write a procedure, SetPartition(S, i), that inputs a set S and a pos. integer i, and
outputs the set of setpartitions of S into i disjoint (nonempty sets). For
example, SetPartition({a,b,c},1); should give {{{a,b,c}}},
SetPartition({a,b,c},2); should give { {{a,b},{c}},{{a,c},{b}},{{b,c},{a}} },
while SetPartition({a,b,c},3); should give { {{a},{b},{c}} }.
Hint: As usual, use recursion, (with option remember!)
Look at the largest element (n:=max(op(S))). It is either a singleton, so you take
a member of SetPartition(S\n,i1) and add to it the singleton {n}, or
it can be joined to one of the already existing sets of any setpartition in
SetPartion(S\n,i);

Write a program Combine(L) that inputs a list of "sets of sets"
L=[S1,S2, ..., Sk] and outputs the set of all possible sets of the
form
s1 union s2 union ... union sk
for all possible s1 in S1, s2 in S2, ..., sk in Sk
For example Combine([{{a,b},{c,d}},{{e,f},{g,h}}]) should output
{ {a,b,e,f},{a,b,g,h},{c,d,e,f},{c,d,g,h}}
Hint: Use recursion, i.e. do Combine([S1,S2,...,Sk]) by using
Combine([S1,S2,...,S(k1)]) and combining with Sk.

Use the above to write RT(S)

Write a procedure RTn(n) that inputs a pos. integer n and outputs all the
rooted labelled trees on {1, ..., n}

Find
[seq(nops(TRn(n),n=1..7))];
Programs done on March 10, 2008, class
mar10.txt,
contains RT(S): a program that inputs a set S and outputs the
set of rooted trees on S (completed by Dr. Z. after class).
Homework for March 10, 2008 (due March 13, 2008)
Note: This is the version of 9:55am, March 11, 2008.
(thanks to Paul Raff for pointing an error in the previous version).
Please discard the previous version of the homework
(in case you already downloaded it).

Very Important: Read carefully
Dr. Z's masterpiece, especially the introduction,
section 1 and section 4.

Look carefully at Combine(L,r) of
mar10.txt,
(finished by me after class), and convince yourself that it indeed
does what it is supposed to do, and hence that RT(S) gives all rooted
labelled trees on S. Note that the edges are now lists of length 2,
and we consider them directed edges so [i,j] means
i>j and [j,i] means j>i. By convention, in a rooted tree, all
the edges point towards the root.
 Write a procedure Weight(a,T) that inputs a
tree, and a letter a, outputs
the product of a[op(edge)] over all the edges of T.
For example,
Weight(a,{[1,2],[2,3]});
should yield
a[1,2]a[2,3]
(Hint: use mul):

Write a program TW(n,i,a) that inputs a pos. integer n,
another pos. integer i in {1,2, ..., n}, and a letter a,
and outputs the sum of Weight(a,t) over all rooted trees
[i,t], rooted at i.
For example,
TW(2,1,a);
should yield a[2,1],
while TW(2,2,a);
should yield a[1,2],
TW(3,1,a) should yield
a[2,1]a[3,1]+a[3,2]a[2,1]+a[2,3]a[3,1] ;

(Correcting an error pointed out by Paul Raff, this is the corrected version of 3/11/08, 9:55am)
Write a program P(n,i,a): that inputs pos. integers n, and i, with
i ≤ n, and a synbol (letter) a, and outputs the polynomial
(in the a[i,j]'s)
(a[i,1]+a[i,2]+ ... +a[i,n])*TW(n,i,a)
(a[1,i]*TW(n,1,a)+a[2,i]*TW(n,2,a)+ ... + a[n,i]*TW(n,n))
(Note that in the ... there is no a[i,i]), Don't
forget to expand at the end.

Using the methodogy of experimental math, of generating data,
looking for patterns, and making a conjecture,
Conjecture an explicit expression for P(n,i,a).

($5 prize, to be divided among all solvers by March 13, 2008)
Prove your conjectured expression for P(n,i,a), using combinatorics!
(no money for other methods).
[Added March 13, 2008: Congratulations to Lara Pudwell,
Andrew Baxter, Paul Raff, Beth Kupin, Emilie Hogan, and
Brian Nakamura for sharing the $5 prize. They all got it right!.
Here are
Lara's solution
and
Baxter's solution .
Programs done on March 13, 2008, class
mar13.txt,
contains the following nifty programs

Sol(n,a): Solves the generic google PageRank's equation for
n webpages and arbitrary probabilities of going from one
webpage to another. (Warning: do not try to apply it to
the full internet!)

MTT(n,a): (The Matrix Tree Theorem).
Inputs a pos. integer n, and outputs the
weight enumerator of all labelled trees on {1, ..., n} rooted
at 1, in case a is a letter, or inputs a numerical list of
lists of size n, that is the adjanency matrix of the graph, and
outputs the number of socalled spanning trees
(trees whose vertices are all the vertices of the graph
({1,2, ..., n}) and whose n1 edges are drawn from the edges of
the graph.

Arthur(n): Implements MTT(n,a) for the complete graph.

Cayley(n): Spells out Arthur(n) for the complete graph,
getting the determinant of a certain matrix, and then
evaluating it.

Cayleynx(n,x): a generalization of Cayley(n) that is easier to
prove (see homework problem below)
Homework for March 13, 2008 (due March 24, 2008)

Find and a prove a closedform expression, in n and x, for Cayleynx(n,x).
Then plugin x=n and deduce Cayley's formual.

Use the Matrix Tree Theorem to write a short program SPBP(m,n), that inputs two integers m and n,
and outputs the number of spanning trees for the complete bipartite graph
K_{m,n} (this is a graph with m+n vertices, m of which are called men,
and n of which are called women, and there are all the possible mn edges between
men and women, but no edges between men and no edges between women).

[I don't the answer for that one, I didn't try]
Output many values for different m and n for SPBP(m,n) and see if they happen
to be in Sloane. Try to conjecture a closed form formula for SPBP(m,n), if possible,
or at least for SPBP(m,1),SPBP(m,2),SPBP(m,3), ... as far as you can.

Consider the graph G(n,i) whose vertices are {0, ..., n1} and any vertex v is connected
to v+1, v+2, ..., v+i, v1, .., vi (mod n) (so there are ni edges).
Write a program,SPW(n,i), that finds the number of spanning trees of G(n,i).

[I don't the answer for that one, I didn't try]
Output many values for different m and n for SPW(n,i) and see if they happen
to be in Sloane. Try to conjecture a closed form formula for SPW(n,i), if possible,
or at least for SPW(n,1),SPW(n,2),SPW(n,3), ... as far as you can.

Carefully read my writeup on the
Lagrange Inversion Formula

Write a Maple program LIF(PHI,t,N) that inputs a function Φ of t, and outputs the
first N coefficients of the formal power series that satisfies the functional equation:
u(t)=t Φ(u(t))

Using Generatingfunctionology, convince yourself that the exponential generating function, T(t), for
labelled rooted trees satisfies the functional equation
T(t)=t e^{T(t)} .
Use the Lagrange Inversion Formula (by purely human means) to give yet another proof of Cayley's
n^{n2} formula.

Use Generatingfunctionology to find the number of ways of
partitioning {1, ..., 100} into a disjoint union of sets, each of
which has a size that is a perfect square.

Use the Lagrange Inversion Formula to find the number of labelled rooted trees
on 100 vertices where the root has two neighbors, and each other vertex has exactly three neighbors,
or exactly one neighbor.

Enjoy the Spring Break, but don't forget to do the homework!
Added March 17, 2008:
Pick a
final project ,
March 17, and March 20 2008
Spring Break.
Programs done on March 24, 2008, class

mar24.txt,
implements the Lagrange Inversion Formula.
It contains the following nifty programs

NoLIFexp(PHI,z,N): inputs an expression PHI in a variable z,
and an integer N, and outputs the first N coeffs. of the
t^n/n! of the formal power series u(t) that satisfies the
Functional Equation
u(t)=t&Phi(u(t))

NoLIF(PHI,z,N): inputs an expression PHI in a variable z,
and an integer N, and outputs the first N coeffs. of the
t^n of the formal power series u(t) that satisfies the
Functional Equation
u(t)=t&Phi(u(t))

LIF(PHI,z,N): Uses the Lagrange Inversion Formula to find
exactly what NoLIF(PHI,z,N) does.

mar24a.txt,
Uses the symbolic (and numeric) Matrix Tree Theorem.
It contains the following nifty programs

LT(n): inputs a pos. integer n (n ≥ 3),
and outputs the set of labelled trees
on {1, ..., n} (all n^{n2} of them!). It uses the
Matrix Tree Theorem in its symbolic manifestation, and the fact
that algebra is combinatorics (and vice versa)

ST(n,G): inputs a pos. integer n (n ≥ 3) and a set of edges G (where an edge
connecting i and j is denoted by {i,j}), representing a labeled
graph on {1, ..., n}, and outputs the set of spanning trees of E.

NuST(n,G): inputs a pos. integer n and a set of edges G (where an edge
connecting i and j is denoted by {i,j}), representing a labeled
graph on {1, ..., n}, and outputs the NUMBER of spanning trees of E.

Cycle(n):
inputs a pos. integer n and outputs the cycle graph on
{1, ..., n}

Wheel(n):inputs a pos. integer n and outputs the wheel graph on
{1, ..., n,n+1}, where the center is labeled n+1.
Homework for March 24, 2008 (due March 27, 2008)

Write a program a(r,N), that inputs pos. integers r and N
and outputs the first N terms of the counting sequence
for labelled rooted trees where the root has degree ≤ r and
every other vertex had degree
≤ r+1 (i.e., viewed from the top, it has ≤ r children).
Look at the sequences for r=1,2,3, ... in Sloane,
and see which exist.

An ordered tree is an unlabelled tree where there is a distinguished
vertex r, and it has children, ordered from left to right, each
of whom may have no children, or start its own dynasty.
Write a program b(r,N), that inputs pos. integers r and N
and outputs the first N terms of the counting sequence
(i.e. the list whose nth term is the number of such trees with n vertices)
for ordered trees where every vertex has
≤ r children.
Look at the sequences for r=1,2,3, ... in Sloane,
and see which exist.

Write a program c(r,N), that inputs pos. integers r and N
and outputs the Nterm sequence (list) whose nth entry is
the number of
ordered trees with n vertices, and where every vertex has
either no children (i.e. is a leaf) or has
exactly r children.
Can you conjecture a closedform formula for c(r,N)? Can you prove it?
(using the Lagrange Inversion Formula or otherwise).

Conjecture a closedform expression for
NuST(n+1,Wheel(n));

Define a generalized wheel, Gwheel(n,r),
with rn+1 vertices
where
the outer rim is labelled 1...n,
the next inner rim is labelled n+1, ..., 2n
...
the innermost rim is labelled (r1)n+1, ..., rn
the center is labelled rn+1.
Assume that within each rim, m has an edge to its immediate neighbors
(so there is an edge between m and m+1 if m is not a multiple of n
and an edge between jn+n and jn+1),
and in addition, the center has an edge to each
the innermost rim, and each other vertex ,m, not on the inner rim
is connected to m+n.
Write a program Gwheel(n,r) that outputs this graph as a set of
edges.

For r=1,2,3, ... (as far as you can go), compute sequences
for the number of spanning trees of Gwheel(n,r) and look them
up in Sloane. If possible, conjecture "nice" expressions for them
and prove them.

Read about Prüfer's bijection in
Dr. Z.'s crash course on enumerative combinatorics, or Wikipedia,
or, in more detail, in almost any combinatorics text, and
program it, first the "easy" direction (from labelled trees to codes),
and then the reverse. Call them S, and T resp .. Check empirically that
ST and TS are the identity mappings on their respective domains.
Programs done on March 27, 2008, class
mar27.txt,
to construct and count integer partitions. It contains the
following procedures.
 Par(n): the set of integerpartitions of n
 Par1(n,k): the set of partitions of n whose largest part is k
 par1(n,k): the number of partitions of n whose largest part is k
 par(n): the number of integerpartitions of n, using a 2parameter recurrence
(i.e. adding up par1(n,k) for k from 1 to n)
 P(N): the first N terms of the partition function (the stupid way, using par(n))
 Pf(N):the first N terms of the partition function
(using the generating function
1/(1q)/(1q^{2} )/(1q^{3})/...)
 p(n): the number of partitions of n using the EulerPentagonal recurrence
 Pff(N):the first N terms of the partition function
(using the Eulerpentagonal recurrence)
Homework for March 27, 2008 (due March 31, 2008)

Write a program, call it Par2(n,k), that inputs pos. integers n and k
and outputs the set of integer partitions with exactly k parts.

Write a program, call it par2(n,k), that inputs pos. integers n and k
and outputs the number of integer partitions with exactly k parts.

Prove that par1(n,k) is always equal to par2(n,k). Construct an
explicit bijection between Par1(n,k) and Par2(n,k), and
vice versa.

Modify Par(n) to write a program cPar(n,d,S) that inputs a pos. integer n, a
pos. integer d, and a set S of integers between 0 and d1 and outputs the
set of partitions of n all of whose parts are congruent to some element
s in S modulo d. For example cPar(n,2,{1}); is the set of partitions of n
all whose parts are odd, cPar(n,5,{1,4}); is the set of partitions of n
all whose parts are congruent to 1 or 4 modulo 5.

Modify par(n) to write a program cpar(n,d,S) that inputs a pos. integer n, a
pos. integer d, and a set S of integers between 0 and d1 and outputs the
number of partitions of n all of whose parts are congruent to some element
s in S modulo d. For example cpar(n,2,{1}); is the number of partitions of n
all whose parts are odd, cpar(n,5,{1,4}); is the number of partitions of n
all whose parts are congruent to 1 or 4 modulo 5.
(In other words, cpar(n,d,S) is the number of elements of cPar(n,d,S).)

Modify Par(n) to write a program dPar(n,d) that inputs a pos. integer n, a
pos. integer d, and outputs the
set of partitions of n where the difference between consecutive (and hence any)
two parts is ≥ d.
For example, dPar(n,0) is just Par(n), while dPar(n,1) is the set of
partitions of n into distinct parts.

Modify par(n) to write a program dpar(n,d) that inputs a pos. integer n, a
pos. integer d, and outputs the
number of partitions of n where the difference between consecutive (and hence any)
two parts is ≥ d.
For example, dpar(n,0) is just par(n), while dpar(n,1) is the number of
partitions of n into distinct parts.
(In other words, dpar(n,d,S) is the number of elements of dPar(n,d,S).)

Prove, empirically, for 0 ≤ n ≤ 200, that
cpar(n,2,{1})=dpart(n,1)

(5 dollars to be divided between all those who get it right by
March 31, 2008, 12:00noon, and who have not seen it before).
Find a bijective proof of the above fact, and program it.
In other words find two mappings
S: cPar(n,2,{1})>dPar(n,1)
T: dPar(n,1)> cPar(n,2,{1})
such that ST and TS are both the identity mappings.

Prove, empirically, for 0 ≤ n ≤ 200, that
cpar(n,5,{1,4})=dpart(n,2)

(100 dollars to the first person to get it, offer expires last day of classes).
Find a nice bijective proof of the above fact, and program it.
In other words find two mappings
S: cPar(n,5,{1,4})>dPar(n,2)
T: dPar(n,2)> cPar(n,5,{1,4})
such that ST and TS are both the identity mappings.
Handout and Programs given on March 31, 2008, class

Handout: Dave Bressoud and Dr. Z's
"proof from the book", of Euler's pentagonal recurrence.

mar31.txt,
generating function for the number of partitions with bounded number
of parts and bouded larget part. It contains
procedures, G(M,N,q), that inputs pos. integers M and N
and a variable q, and outputs the polynomial in q, whose
coeff. of q^{n} gives the number of partitions of n
into at most M parts, each of which is ≤ N, and
G1(M,N,q), the Gaussion polynomial
[(1q^{M+1})(1q^{M+2})... (1q^{M+N}) ]/
[(1q)(1q^{2}) ...(1q^{N}) ]
(that is allegedly equal to it).

Courtesy of Andrew Baxter:
Baxter's neat Ferrers Diagrams package, that
would help visualize partitions and planepartitions.
First Homework Set for March 31, 2008 class, Due April 1, 2008
[No extensions!]

Recall that for any set of nonnegative integers A, mex(A) is
the smallest nonnegative integer not in A. For example,
mex({2,4,5})=0, mex({0,1,2,5,8})=3, etc.
Define a sequence a_{i} recursively by
a_{1}=2, and for i ≥ 1 by:
a_{i}=mex({0,1} U { j a_{r} ,
j ≥ 1 , 1 ≤ r < i}) ,
Prove the following properties of a_{i}

There are infinitely many i such that a_{i+1}a_{i}=2

Every even intger n ≥ 6 can be written as
a_{i}+a_{j}, for some pos. integers i and j.

Define a sequence F(n) by,
F(a_{i1} a_{i1}
a_{i2} ...a_{ir})=(1)^{r}
if n can be expressed as a product of distinct
a_{i}'s , and 0 otherwise.
Let
G(n)=add(F(i), i=0..n)
Prove that G(n) ≤ Cn^{.999} , for some fixed constant C.

Remember that Euler's pentagonal product
η (q)=(1q)(1q^{2})(1q^{3}) ... ,
when expanded, has lots of 0coefficients and the rest are 1 or 1.
Condider the 24th power of that
η (q)^{24}=[(1q)(1q^{2})(1q^{3}) ... ]^{24},
and let's call the coeff. of q^{n}, τ(n). Prove that
τ(n) is never zero.
Second Homework Set for March 31, 2008 class, Due April 3, 2008

Write a Maple implementation of the BressoudZ
involution from the book.

Write a procedure, PP2a(a1,a2,N,q), that inputs two non.neg.
integers a1,a2 such that a1 ≥ a2 ≥ 0, and outputs
the generating function for Ncolumned and
2rowed plane partitions whose
leftmost column is [a1,a2].

Write a procedure, PP2(M,N,q), that inputs two non.neg.
integers M and N and outputs the generating function for
all 2rowed and Ncolumned plane partitions, all of whose
parts are ≤ M.

Write a procedure, PP3a(a1,a2,a3, N,q), that inputs three non.neg.
integers a1,a2,a3, such that a1 ≥ a2 ≥ a3 ≥ 0, and outputs
the generating function for Ncolumned and
3rowed plane partitions whose
leftmost column is [a1,a2,a3].

Write a procedure, PP3(M,N,q), that inputs two non.neg.
integers M and N and outputs the generating function for
all 3rowed and Ncolumned plane partitions, all of whose
parts are ≤ M.

Conjecture explicit expressions for PP2(M,N,q) and PP3(M,N,q) .

(5 dollars to be divided among the people who would get it right).
Conjecture an explicit expression for PPK(M,N,q), the generating
function for Krowed, Ncolumned Plane Partitions each of whose
parts is ≤ M .
Programs done on April 3, 2008, class
apr3.txt
contains the following procedures:

GuessPolq(L,q,z): inputs a list L of polynomials in the variable q, and
a variable z, and tries to guess a polynomial P(z) such that
P(q^n)=L[n]

PP2(M,N,q):
The generating function for plane partitions whose 3D Ferrers graph
is contained in an M by N by 2 box. Equivalently, the weightenumerator of 2 by M arrays
a[i,j], i=1..2, j=1..M, of nonnegative integers,
weakly decreasing along rows and columns
(according to the weight q^{sum of entries})

PP3(M,N,q):
The generating function for plane partitions whose 3D Ferrers graph
is contained in an M by N by 3 box. Equivalently, the weightenumerator of 3 by M arrays
of nonnegative integers
a[i,j], i=1..3, j=1..M weakly decreasing along rows and columns
(according to the weight q^{sum of entries})
Homework for April 3, 2008 class, Due April 7, 2008
 Using GuessPolq, conjecture a closed form formula (as a product) for
PP2(M,N,q) and PP3(M,N,q). Generalize this to conjecture the generating
function for "PPK(M,N,q)", i.e. the weightenumerator for plane partitions
whose 3D Ferrers diagram is inside an M by N by K box.
formula (as some kind of product)

Write PPK(M,N,K,q) that generalizes PP2 and PP3 to plane partitions with K rows.
Check its output for K=4,5 against your conjecture.

Plugin M=N=K=infinity in the above prodcutformula and deduce MacMahon's expression
for ALL plane partitions
1/[(1q)(1q^{2})^{2}(1q^{3})^{3} ... ]

Write a program Solid22(N,M,q) that inputs integers M and N and outputs the
generating function for all solid partitions whose 4D Ferrers graph fits
inside a 2x2xMxN (fourdimensiona) box. In other words, all
3D 2x2xM arrays
a[i,j,k], 1 ≤ i,j ≤ 2, 1 ≤ k ≤ M
with 0 ≤ a[i,j,k] ≤ N such that a[i,j,k] is weakly decreasing
in every one of the three direction.
Hint: First write a program Soild22a(a11,a12,a21,a22,M,q) that
computes the generating function according to the information about the top layer, and then, to get
Solid22(N,M,q) simply do
Soild22a(N,N,N,N,M+1,q)/q^{4N}
Note that a11 ≥ a12, a11 ≥ a21, a21 ≥ a22
in addition to them being between 0 and N.
Programs done on April 7, 2008, class
apr7.txt
contains the following procedures:

A(n,S): inputs a pos. integer n and a set of patterns S and outputs
the set of permutations on {1, ..., n} that do not contain any
of the patterns of S. For example, A(6,{[1,2,3],[1,3,2]}); gives
the set of permutation pi of {1,2,3,4,5,6} such that you never have
pi[i1] < pi[i2] < pi[i3]
and never
pi[i1] < pi[i3] < pi[i2]
for 1 ≤ i1 < i2 < i3 ≤ 6

SeqSN(S,N): Inputs a set of patterns S and a pos. integer N, and outputs
the sequnce of length N whose nth element is nops(A(n,S)).
Homework for April 7, 2008 class, Due April 10, 2008

Prove rigorously (but humanly) that A(n,{[1,2,3],[1,3,2]})=2^{n1} .

Try to prove that both A(n,{[1,2,3]}) and A(n,{[1,3,2]}) are given by the Catalan numbers
C_{n}=(2n)!/(n!(n+1)!) .

Look at all the binomial(6,3)=20 sets that have exactly three patterns of length three .
try to conjecture expressions for A(n,S) for as many S as you can.

Extend A(n,S) and write a procedure B(L,S) that inputs a list L of nonneg. integers
and outputs all words (i.e. lists ) with L[1] 1's, L[2] 2's, ... L[n] n's that avoid
the set of patterns S. Note that B([1$n],S) should equal A(n,S).
Warning, this is very slow, so only try it for L whose sum is ≤ 7 .
Programs done on April 10, 2008, class
Today was such a nice day, so the class was on the lawn,
using our biological computers between our shoulders, aided by
memorystorage device called pencilandpaper.
We talked about Young tableaux and how to count them.
The following short program:
apr10.txt,
contains programs Par(n) to generate all integerpartitions of n, and
GuessPol(L,n) that inputs a list L of numbers and a symbol n, and
outputs a polynomial P, of degree nops(L)4, if it exists, such that P(i)=L[i], for i=1..nops(L)
(or returns FAIL). These are needed for the homework problems.
Homework for April 10, 2008 class, Due April 14, 2008

Write a program: Children(L) that inputs a partition of L, and outputs
the of partitions obtained by reducing by one any of its parts, such
that the remaining list is still a partition. For example,
Children([3,2,2,1]); should give the set {[2,2,2,1],[3,2,2]}, but does not include
[3,1,2,1]). Rememeber that if you get [3,2,2,0] you must kick out the 0 at the end.

Write a program f(L) that inputs a partition L and outputs the
number of Standard Young tableaux of shape L.
Hint: Using the "going down" recurrence f(L)=Sum(f(L), L in Children(L));,
together with the the initial condition f([])=1 .

Verify up to n=13, the Humberto conjecture that
Sum(f(L)^2, L in Par(n)) =n!

Write a procedure for
a(n):=Sum(f(L), L in Par(n))
and look up
seq(a(n),n=1..9)
in Sloane.

Using GuessPol in
apr10.txt,
conjecture explicit expressions for
 f([a,1]); (a ≥ 1)
 f([a,2]); (a ≥ 2)
 f([a,3]); (a ≥ 3)
 f([a,4]); (a ≥ 4)
(Warning: the lists that GuessPol takes start at the argument being 1, here
the argument starts later.

Glancing at the above output, conjecture an explicit expression for
f([a,b]), with a ≥ b ≥ 0.

(5 dollars to be divided among all correct solvers).
Using the same methodololgy as above conjecture an explicit expression for
f([a,b,c]) for a ≥ b ≥ c ≥ 0 .
Then Rigorously prove it!

(6 dollars to be divided among all correct solvers).
Conjecture an expression for f([a1,a2, ..., ak]) for arbitrary k.

(7 dollars to be divided among all correct solvers)
Prove the above conjecture.
Programs done on April 14, 2008, class
apr14.txt,
contains the following procedurs (in addition to Par(n) from last time and GuessPol(L,n)
described last time)

Mamas(L): inputs a partition L and outsputs all its "parents"
(what we called children last time), i.e. all the shapes obtained from L
by nibbling (legally) one box.

f(L) : inputs a partition L and outputs the number of Standard Young Tableaux of
shape L.

Guess2(a,b1) :
inputs a symbol a and a pos. integer b1, and outputs the
guessed polynomial, in a, for f([a,b1]);

Guess3(a,b1,c1) :
inputs a symbol a and a pos. integers b1,c1, with b1>=c1>0 and outputs the
guessed polynomial, in a, for f([a,b1,c1]);
Homework for April 14, 2008 class, Due April 17, 2008

Recall that a rational function f(x) is a quotient of polynomials f(x)=P(x)/Q(x).
Write a program GuessRat1(L,x,d) that inputs a list L, a variable x, and a non.neg. integer d
such that 2*d+5 ≤ nops(L), and outputs a rational function P(x)/Q(x) with
degree(P,x), degree(Q,x) ≤ d, such that P(i)/Q(i)=L[i] for all i from 1 to nops(L),
if one exist, and FAIL otherwise.

By trying d=1..(nops(L)5)/2, write a program GuessRat(L,x)
that guesses a rational function P(x)/Q(x) that fits L.

By using GuessRat, conjecture a rational function for
g2(a,b):=f([a,b])/((a+b)!/(a!b!)) .
Hint, first, freeze b, and get rational functions g(a,b1) for various b, and then
use GuessRat with respect to that list to conjecture an expression for g(a,b).

By using GuessRat, conjecture a rational function for
g3(a,b,c):=f([a,b,c])/((a+b+c)!/(a!b!c!)) .
Hint, first, freeze c, and b, to get rational functions g(a,b1,c1) for various b1, and c1.
etc.

Write a procedure Children(L) (with the new meaning of children), that inputs the set of partiotions
obtained from L by legally adding one box (i.e. the set of partitions L1 such that L is a member
of Mamas(L1)).

What can use say about
h(L):=Sum(f(L1), L1 in Children(L))/f(L) .
Programs done on April 17, 2008, class
apr17.txt,
contains the following procedurs (in addition to those of apr14.txt
described above)

Children(L): Inputs a partition L and outputs the set of
partitions obtained from L my legally adding one box to each qualified row.

Y(n,k): Inputs positive integers n and k and outputs Sum(f(L)^k, L in Par(n)),
where f(L) is the number of Standard Young Tableaux of shape L.
Y(n,2) should equal n!

SeqY(N,k): the sequence of Y(n,k) for n=1.. N

Nes(n): verifying the "goingup recurrence" satisfied by L for all L in Par(n)
(n+1)f(L)=Sum(f(L1), L1 in Children(L))
Yr(n,k,r): Inputs positive integers n, k, and r, and outputs Sum(f(L)^k, L in Par(n,k)),
where f(L) is the number of Standard Young Tableaux of shape L.
Homework for April 17, 2008 class, Due April 21, 2008

Write a procedure, SYT(L), that inputs a partition L, and outputs the set of
Standard Young Tableaux of shape L. For example
Y([2,2]); should yield {[[1,2],[3,4]],[1,3],[2,4]]}.

A SemiStandard tableau of shape L is a way of fillingin L with
positive integers such that it is strictly increasing along the columns
but only weakly increasing along the rows. Write a program SSYT(L,r) that
inputs a shape L and a pos. integer r, and outputs the set of
SemiStandard Young tableaux of that shape that only uses the
integers {1, ..., r}. For example SSYT([2],2); should
return {[1,1],[1,2],[2,2]}; while SSYT([1,1],2); should return
{[1,2]};
Hint: write the analog of Mamas, by considering those tableaux obtained
by deleting the largest integer, r.

(10 dollars to be divided among those that get it right).
Use human ingenuity to prove the "GoingUp recurrence",
(n+1)f(L)=Sum(f(L1), L1 in Children(L))
by using
induction, together with the "GoingDown recurrence" (that is obvious):
f(L)=Sum(f(L1), L1 in Mamas(L))
Programs done on April 21, 2008, class
apr21.txt,
contains the following procedurs (in addition to those of apr17.txt
described above)

F(L): inputs a partition L (given as a list of weakly decreasing pos. integers)
and outputs the set of Standard Young Tableaux of shape L. For example,
F([2,2]); should give {[[1,2],[3,4]],[[1,3],[2,4]]} .
 PY(Y): inputs a tableaux and prints it nicely
 PSY(S): prints out nicely all the tableaux of the set S
 YFF(L): The Rowland conjectured explicit formula for the number of SYT of shape L
(unfortunately already proved, about one hundred years ago, by Young, Frobenius, and
MacMahon)
 ProveYFF(k): inputs a pos. integer k, and outputs true or false according to
whether YFF(L) is true for all (infinitely many!) Young tableaux with ≤ k rows.
(Warning for k ≥ 7 it is starting to get slow, and for general k, you still need humans).
Homework for April 21, 2008 class, Due April 24, 2008

A skewshape is a pair of partions [L1,L2] such that the Young diagram of L1 is
a subset of the Young diagram of L2. A skewstandardYoung tableau, is
a way of fillingin the integers 1 through nops(L2)nops(L1) such that
they are increasing along rows and along columns. Modify f(L) and F(L) to
write procedures fSkew(L1,L2) and FSkew(L1,L2) that computes their
numbers and the set itself, respectively.

Given a partition (shape) L, label its boxes like you would the entries
of a matrix, i.e. box (i,j) is the jth box of the ith row.
The hooklength of a box (i,j) in a shape L, is the number
of boxes to its right (in the ith row) plus the number of boxes below it
(in the jth column) plus 1 (for itself). For example, the hook length
of box (2,3) in the shape [6,5,4,3,3,3] is 7.
Write a procedure Hook(L,i,j) that inputs a partition L and pos. integers i and j,
and outputs the hooklength of box (i,j) in the shape L. For example,
Hook([6,5,4,3,3,3],2,3);
should return 7. It should return FAIL if box (i,j) is not a member of the shape L.

Write a prodedure HLF(L), that inputs a partition L and outputs
n! divided by the product of all the hooklengths.
(where n is the number of boxes in L (i.e. L[1]+L[2]+ ...+L[nops(L)]).

Verify empirically that HLF(L)=YFF(L) for all partions L of n ≤ 10 .

[5 dollars to be divided between all correct answers].
Prove that HLF(L)=YFF(L) for every shape.

[7 dollars to be divided between all correct answers]
Prove that f(L)=YFF(L) for shape with k rows, with k arbitrary.
Hint: You may want to use a variant of the Lagrange Interpolation Formula.
Programs done on April 24, 2008, class
apr24.txt,
contains the following procedurs (in addition to those of apr21.txt and sooner
described above)

HL(L,i,j): inputs a partition L and pos. integers i and j and outputs
the hooklengths of box (i,j) in L

HLF(L): The hooklength formula for the number of Standard Young Tableaux of shape L
(equals YFF(L)).

Ins1(Y,i,row1): one step of one step of the RobinsonSchenstead algorithm.
Inputs a Young tableau Y, and an integer i, and a row number row1, and
outputs another Young tableau and a bumpee.


Ins(Y,i): one step of the RobinsonSchenstead algorithm.
Inputs a Young tableau Y, and an integer i, not in Y, and
outputs another Young tableau with one extra box added, and the
rownumber of that added box.
[corrected, 4/25/08 by Slava and Humberto]
Homework for April 24, 2008 class, Due April 28, 2008

Complete the RobinsonSchenstead algorithm,
writing a procedure, RS(pi), that inputs a permutation pi of
{1, ..., n} and outputs a pair of Standard Young Tableaux of the
same shape (each with n boxes). Do it by setting Y:=[[pi[1]]], and
then doing for each i=2..n, in turn, Y1:=Ins(Y1,pi[i]). The final
tableau is the Insertion tableau. To get the second tableau (the socalled
template tableau) keep inserting, in, turn, 1, 2, 3, ..., n, where i goes
to the box that was added right after the insertion of pi[i].

Write the inverse of Ins(Y,i), call it Del(Y,bumpee).
(Hint: first write the inverse of Ins1)

Using the above, write a procedure, IRS(Y1,Y2), that inputs two Standard
Young tableaux Y1,Y2, of the same shape (first check that fact, and
return FAIL if it fails), and outputs a permutation pi, such that
RS(IRS(Y1,Y2))=(Y1,Y2).

Write a procedure that inputs a permutation and outputs its inverse.

How are RS(pi) and RS(inverse(pi)) related?
Programs done on April 28, 2008, class
apr28.txt,
contains the following procedurs (in addition to those of apr24.txt and sooner
described above)

RS(pi): performs the RobinsonSchenstead Correspondence to a permutation pi

TestRS(n): tests that the RS(pi) ranging over all pi, does indeed gives the
set of pairs of SYT of the same shape with n boxes.
Homework for April 28, 2008 class, due May 1, 2008

A semistandard Young tableaux (also called columnstrict) is a way of filling
an rrowed shape with the integers {1, ..., n} (n ≥ r), such that
the entries are weakly increasing along the rows and strictly increasing
along the columns. Write a program SSYT(L,n) that inputs
a shape L and a pos. integer n, and outputs the set of semistandard Young tableaux.

The weight of a tableaux (a[i,j]) is the product of x[a[i,j]] over all
entries. For example, the weight of [[1,2,2,3],[2,3,3,4],[3,4,5]] is
(x[1]x[2]x[2][x[3])(x[2]x[3]x[3]x[4])(x[3]x[4]x[5]).
Let S(L,n,x) be the sum of the weights of all members of SSYT(L,n).
Write a procedure Sslow(L,n,x) to find the polynomial (in x[1], ..., x[n])
S(L,n,x)

Write a faster program . Sfast(L,n,x), with the same output as Sslow(L,n,x), but
without first literally constructiong the set SSYT(L,n). Do it by
establishing a recurrence that expresses S(L,n,x) in terms of S(L',n1,x) for smaller L' s.

[2 dollars to be divided by the k solvers].
Conjecture an explicit expression for S([a,b],2,x)*(x[1]x[2]), as a polynomial in
(with powers that involve a,b), for a ≥ b > 0

[3 dollars to be divided by the k solvers].
Conjecture an explicit expression for S([a,b,c],3,x)*(x[1]x[2])*(x[1]x[3])*(x[2]x[3]), as a polynomial in
x[1],x[2],x[3] (with powers that involve a,b,c)
a ≥ b ≥ c > 0

[10 dollars to be divided by the k solvers].
Conjecture an explicit expression for
S([a1,a2, ..., an],n,x)*Product(Product(x[i]x[j], j=i+1..n),i=1..n)
as a polynomial in x[1],x[2],x[3], ..., x[n] (with powers that involve a1,a2, ..., an)
a1 ≥ a2 ≥ ... ak > 0

In preparation for Polya theory, to be covered next time, carefully read pages
10 and 11 of:
Dr. Z.'s Crash course on enumerative combinatorics
Programs done on May 1, 2008, class
may1.txt,
contains the following procedurs, starting PolyaRedfield theory.

AvF(G): The average number of fixed points of the group of permutations G.

kefel(pi,sig): permutation pi times permutation sig (as permutations)

GenG(S): Given a set of permutations S all of the
same size, outputs the group of permutations
generated by them
Homework for May 1, 2008 class, due May 5, 2008

Figure out the group of rotations of the three dimensional cube that send faces
to faces (hint: there 24 of them). Label the top 1, the bottom 2, the left 3, the
right 4, the front 5, and the back 6.

Write a procedure Necklace(n), that inputs an integer n, and that outputs
the group of rotations of an nbead necklace, as a group of permutations of
1, 2, ...,n , where the beads are arranged clockwise.

Write a procedure Gp(G,n), that inputs a graph G with vertices {1, ...n},
given in terms of a set of edges, and outputs the set of permutations pi
of {1,...,n} that when pi is applied to G, the set of edges remain the same.
For example Gp(3,{{1,2},{1,3}}); should output {[1,2,3],[1,3,2]}.

Write a procedure Cycle(pi), that inputs a permutation pi, and outputs the set of its
cycles. A cycle i1>i2>i3>...>ik>i1 should be denoted by [i1,i2, ..., ik]
For example Cycles([1,2,3]); should return {[1],[2],[3]}, and
Cycles([3,2,1]); should return {[1,3],[2]}.
Programs done on May 5, 2008, class
may5.txt,
Finishing PolyaRedfield theory.
Have a great summer!
Dr. Z.'s teaching page