# AnalyticComb

Documentation for AnalyticComb.

# References

This package implements solutions for combinatorial problems using analytic combinatorics. Check the text book by Flajolet & Sedgewick and Coursera's full course by Robert Sedgewick.

Kudos to Ricardo Bittencourt for his introductory texts on the subject and for helping in an initial implementation.

# Background

In 1751, Euler was studying the number of ways in which a given convex polygon could be decomposed into triangles by diagonal lines.^{[1]}

He realized that the progression of numbers in the solution (1, 2, 5, 14, 42, 132,...) was directly related to the coefficients of the series expansion of the polynomial fraction $\frac{1−2a−\sqrt{1−4a}}{2aa}$, that is: $1+2a +5a^2 + 14a^3 + 42a^4 + 132a^5 + ...$

Given any constructable combinatorial structure, one can use a set of operators to find a generating function and then approach the problem analytically.

# Introduction

For newcomers, this an analytic approach to combinatorial problems. Modelling this type of problem often relies on intuitive arguments. The symbolic method describe such situations with a grammar of operators: Sum, Cartesian product, Sequence, Multiset, Powerset and Cycle. Such operators yield an algebraic expression (e.g. $P(z)$), called the generating function, which is directly related to the problem via complex analysis. We are generally interested in the coefficients of its series expansion. That is, let the series expansion of $P(z)$ be $T(P(z)) = \sum_{n=1}^{\infty} a_n x^n$. Then, the values of $a_n$ correspond to the counts of objects of size $n$ in this combinatorial class.

For instance, the number of binary words (e.g. abababbabab...) of size n is given by $W_n = 2^n$. Using the sequence operator ($SEQ(A) \implies A(z) = \frac{1}{1-z}$) , we find the generating function:

$W = SEQ(Z+Z) \implies W(z) = \frac{1}{1 - 2z}$. $T(W(z)) = 1 + 2z + 4z^2 + 8z^3 + ...$.

This approach can be used to solve complex problems in a systematic way.

- 1Flajolet, P., & Sedgewick, R. (2009). Analytic combinatorics. Cambridge University press. Page 20