## Instructor Graham Denham WF, 2:30-3:30pm MWF 11:30-12:30pm MC 107 Introductory Combinatorics, 5th edition, Richard Brualdi, available at the bookstore 0.5 course from: Mathematics 2120A/B, 2155A/B, 2211A/B, Applied Mathematics 2811B, or permission of the Department. October 25 and 27 (two parts; in class) December 16th, 9am-noon; HSB 11. 40% Final exam; 30% midterm; 30% assignments

## Reading

Wilf's book on generating functions is a good secondary reference, available for free here.  The course synopsis below will contain other specific suggestions as the course progresses.

## Synopsis

A week-by-week record of what's going on:
• 9-8: Introduction and overview. Our focus will be mostly on counting. What, though, constitutes a solution to a counting problem?
• 9-11, 9-13, 9-15: permutations and combinations.  We can organize some classical counting problems in terms of additive and multiplicative principles.  $$k$$-element sequences of entries from $$[n]=\{1,2,...,n\}$$ can also be thought of as functions $$f\colon[k]\to [n]$$.  From this point of view permutations are the injective functions.  The discussion extends to permutations of multisets, which we see can be counted by multinomial coefficients.  Subsets of $$[n]$$ are in one-to-one correspondence with permutations of a multiset with just two kinds of element.  This gets us back to classical binomial coefficients.
• 9-18: the pigeonhole principle 9-20: fancy variations and applications like the Erdos-Szekeres Theorem
• 9-22: generating permutations: the Johnson-Trotter algorithm.
• 9-25: finding your place in the Johnson-Trotter order.  Inversions in permutations: recovering a permutation from its inversion sequence.
• 9-27: enumerating subsets: we looked at subsets of $$[n]$$ as n-digit binary sequences.  By writing subsets as "words" with digits in decreasing order, we saw that the counting order and the lexicographic order were the same.
• 9-29: Gray codes and enumerating subsets of a fixed size.
• 10-2: if we express a subset of $$[n]$$ as a word by writing the elements in decreasing order, then the counting order agrees with the lexicographic order.  If we restrict our attention to subsets of a fixed size, we saw that it's fairly easy to say where, in order, a given subset appears: if $$\{a_1,a_2,\ldots,a_r\}$$ is a subset and $$a_r>a_{r-1}>\cdots>a_1$$, then the number of subsets that comes before this one in order equals
${a_r-1\choose r}+{a_{r-1}-1\choose r-1}+\cdots+{a_2-1\choose 2}+{a_1-1\choose 1}.$
A recommended exercise is to see what happens to this order if we try some variations: for example:
• write subsets as words in increasing order instead;
• reverse the "alphabetical order" by declaring larger numbers come before smaller numbers instead;
• replace subsets by their complements;
• consider the counting order in reverse.
• 10-4: properties of binomial coefficients: they count something, they are defined by a recurrence relation, and we can find identities that they satisfy, either by combinatorial or algebraic means.
• 10-6: Binomial coefficients are log-concave.  Newton's binomial theorem, and the multinomial theorem.
• 10-16: the Principle of Inclusion-Exclusion allows us to count (finite) sets determined by overlapping conditions.
• 10-18: more examples of the P.I.E., including counting subsets of a multiset, and derangements.
• 10-20: asymptotics of derangements, and permutations with no consecutive $$i,i+1$$'s.
• 10-23, 25, 27: review and midterm
• 10-30: generating functions: the idea that a sequence indexed by nonnegative integers can be packaged and treated as a single object.  We see that the various instances of the binomial theorem are already examples.
• 11-1: one can use (ordinary) generating functions to solve linear recurrence relations.  Products of sums of the form $$1+t+t^2+\cdots+t^{a_i}$$ give generating functions for the number of combinations of a multiset $$\{a_1\cdot 1, a_2\cdot 2, \ldots, a_n\cdot n\}$$.
• 11-3: the expansion of $$1/(1-(X+Y+Z))$$ can be viewed as a formal sum of all words made from the symbols $$X$$, $$Y$$, $$Z$$, and more generally.  This sometimes lets us write down a generating function to solve a counting problem directly.
• 11-6, 8: exponential generating functions.  Products of exponential generating functions have a nice interpretation.  We can count "words" in a set of letters with various conditions on the number of each letter by multiplying e.g.f.s.
• 11-10: composition of exponential generating functions.  One place to read more about exponential generating functions is sections 3.1-3.5 of the book by Herbert Wilf, linked above.
• 11-13, 15: Catalan numbers turn out to be the solution to an amazing number of equivalent counting problems.  They satisfy a quadratic recurrence relation that lets us compute the explicit formula $$C_n=\frac{1}{n+1}{2n\choose n}$$.
• 11-17: Stirling numbers of the second kind $$S(n,k)$$ count the number of partitions of $$[n]$$ into $$k$$ nonempty, unordered blocks.  They satisfy a Pascal's-triangle-type recurrence relation, and the generating function $\sum_{k,n\geq0}S(n,k)t^k x^n/n!$ simplifies beautifully.
• 11-20, 22: (Unsigned) Stirling numbers of the first kind $$s(n,k)$$ count the number of permutations of $$[n]$$ with exactly $$k$$ cycles.  They also satisfy a Pascal's-triangle-type recurrence relation and have a nice generating function, as well as an inverse relationship with the $$S(n,k)$$'s.
• 11-24: The Eulerian number $$e_{n,k}$$ is the number of permutations of $$[n]$$ with exactly $$k-1$$ descents.  The Eulerian numbers satisfy a fourth Pascal's-triangle-type recurrence relation.  The Eulerian polynomials $$A_n(t)$$ satisfy the formula $\sum_{k\geq}k^nt^k=\frac{A_n(t)}{(1-t)^{n+1}}.$
• 11-24,27: If $$p_n$$ denotes the number of partitions of an integer $$n$$, we find $\sum_{n\geq0}p_nt^n=\prod_{k\geq 1}(1-t^k)^{-1}.$  Discussing this generating function leads to Euler's Pentagonal Numbers Theorem.
• 11-29: permutation groups
• 12-1: cycle types, orbits, stabilizers all leading up to Burnside's Lemma
• 12-4: applications of Burnside's Lemma to counting colourings up to symmetry (textbook, 14.2)
• 12-6: Pólya theory (textbook, 14.3) refines Burnside's Lemma to count colourings up to symmetry, keeping track of the number of times each colour appears.

## Syllabus

This is an intermediate course in enumerative combinatorics, the study of counting.  There are not many formal prerequisites, but you will enjoy the course best if you have some enthusiasm for problem-solving and hands-on math. We will review the basics -- how to count permutations and combinations of labelled and unlabelled objects.  We will see how to use formal power series (also known as generating functions) to solve counting problems easily and systematically.  The usual topics include:
• the pigeonhole principle
• permutations and combinations of sets and multisets
• introducing formal power series
• ordinary and exponential generating functions
• solving recurrence relations
• counting graphs and trees
• Joyal's theory of species
• counting in the presence of symmetry (Polya theory) or Lagrange inversion

## Assignments

Learning the art of counting requires, above all, practice.  Accordingly, there will be regular homework assignments.  This is the most important part of the course.  Please note that no late assignments will be accepted.  See the homework page for an up-to-date list.

Some of the assignment problems will be routine, and some will take some thought.  Collaborating with other people can add a lot to the experience of doing math, and I encourage you to do so.  (Research-level mathematics can be done alone, but is probably more often done in groups of two or three.)  Just make sure to write your own solutions, your own way, and to acknowledge any debts you may have.  Ask me if in doubt, since presenting the work of others as your own constitutes a serious academic offence.

Sometimes it can be useful to use some symbolic computation software, for example to evaluate a few terms of a power series.  Try Maple or Mathematica, if you have access or familarity.  You can also use Sage, an open-source symbolic computation tool, online and for free.  For example, create a Sage notebook, and enter the following:
var('t')
f = e^(e^t-1)
f.taylor(t,0,10)

This will give you the first ten terms of the exponential generating function for the Bell numbers, which we will learn about in early November.

## Exams

There will be one midterm which we will schedule at the start of week 2.  Here are some extra problems to practice at the end of the course.

## Math 9043a

The MSc version of this course includes slightly different homework problems, and an additional self-directed written project, to be chosen at the start of term. In this case, the evaluation is weighted as 30% final exam; 25% midterm; 25% assignments; 20% project.  The project is due on the first Monday after the last lecture.

## Further information

Academic dishonesty: Scholastic offences are taken seriously and students are directed to read the official policy

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