# Ordinary Generating Functions

This is a reference document on Ordinary Generating Functions by Fengyang Wang. It covers a subset of the material of MATH 239, MATH 249, or CO 330.

## Sequences

The set of natural numbers, including zero, is important in combinatorics. We denote it by $\mathbf{N} \cup \{0\} = \{0, 1, 2, \ldots\}$.

A sequence $(a_n)_{n\ge0}$ is a function $\mathbf{N} \cup \{0\} \to \mathbf{C}$. We denote by $a_k$ the result of evaluating this function at natural number $k$.

An example of a sequence is the triangular numbers $(T_n)_{n\ge0}$, which counts the number of edges in a complete graph of order $n$. We have, for example, $T_0 = 0$, $T_1 = 1$, $T_2 = 3$, $T_3 = 6$, $T_4 = 10$.

A common way to describe a sequence is by an explicit closed form, which gives the function directly. For the aforementioned sequence of triangular numbers, there is a simple closed form:

$T(n) = \frac{n(n+1)}{2}$

Sometimes, a closed form may not be easy to find or may not be feasible to compute, and we instead can use a recurrence relation. A recurrence relation describes, recursively, the values of the sequence. For the triangular numbers, we can find the following recurrence relation:

$T(n) = \begin{cases} 0 & n = 0 \\ T(n-1) + n & n > 0 \end{cases}$

A third representation of a sequence, which is often more compact and sometimes more computationally useful than the two mentioned here, is by analogy with a power series. Recall (MATH 138 / MATH 148) that a power series is a generalization of a polynomial, with arbitrarily large powers of the formal variable $x$. In general, we may write power series as

$G(x) = g_0 + g_1 x + g_2 x^2 + \dots$

or, using summation notation, as

$G(x) = \sum_{n\ge0} g_n x^n$

Note, in particular, that the coefficients $(g_n)_{n\ge0}$ form a sequence. We may exploit this similarity between sequences and power series to help describe sequences through their analogous power series. For instance, the triangular numbers are described by the power series

$T(x) = \sum_{n\ge0} T_n x^n = \sum_{n\ge0} \frac{n(n+1)}{2} x^n$

Importantly, this power series admits a simple form:

$T(x) = \frac{x}{(1-x)^3}$

It is this compact format that makes generating functions useful.

## Enumeration

Let $S$ be a set. Often, we want to enumerate the elements in $S$ based on some particular metrics. One common kind of metric is to assign to each element of $S$ some non-negative integer. We call this kind of metric a weight function.

More formally, a weight function for $S$ is a function

$w: S \to \mathbf{N} \cup \{0\}$

A common task in enumeration is to count how many objects in $S$ have weight $n$; that is, to find the size of the set

$\{a\in S : w(a) = n\}$

and especially to solve this problem in general for all $n\in\mathbf{N}\cup\{0\}$. Furthermore, it is often convenient to encode this information into a compact, algebraically useful form, if possible.

Generating functions enable us to do this. We can encode all the information above as coefficients for some power series. That is, we can define

$G(x) = \sum_{a\in S} x^{w(a)}$

to be the (single-variable) generating function for weight function $w$ over $S$.