Symbolic method (combinatorics): Difference between revisions

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{{Short description|Mathematical technique}}

{{About|the method in analytic combinatorics|the method in invariant theory|Symbolic method}}

In [[combinatorics]], the '''symbolic method''' is a technique for [[enumerative combinatorics|counting combinatorial objects]]. It uses the internal structure of the objects to derive formulas for their [[generating function]]s. The method is mostly associated with [[Philippe Flajolet]] and is detailed in Part A of his book with [[Robert Sedgewick (computer scientist)|Robert Sedgewick]], ''[[Analytic Combinatorics]]'', while the rest of the book explains how to use [[complex analysis]] in order to get asymptotic and probabilistic results on the corresponding generating functions.

During two centuries, generating functions were popping up via the corresponding recurrences on their coefficients (as can be seen in the seminal works of [[Daniel Bernoulli|Bernoulli]], [[Leonhard Euler|Euler]], [[Arthur Cayley]], [[Ernst Schröder (mathematician)|Schröder]],

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:<math>\frac{g(z)^n}{|G|}.</math>

We are able to enumerate filled slot configurations using either PET[[Pólya enumeration theorem]] in the unlabelled case or the labelled enumeration theorem in the labelled case. We now ask about the generating function of configurations obtained when there is more than one set of slots, with a permutation group acting on each. Clearly the orbits do not intersect and we may add the respective generating functions. Suppose, for example, that we want to enumerate unlabelled sequences of length two or three of some objects contained in a set ''X''. There are two sets of slots, the first one containing two slots, and the second one, three slots. The group acting on the first set is the full symmetric group <math>E_2S_2</math>, andwhich in symbolic combinatorics is traditionally denoted <math>E_2</math>. The group acting on the second slotset is, analogously, <math>S_3 = E_3</math>. We represent this by the following formal [[power series]] in ''X'':

:<math> X^2/E_2 \; + \; X^3/E_3 </math>

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:<math> X/C_1 \; + \; X^2/C_2 \; + \; X^3/C_3 \; + \; X^4/C_4 \; + \cdots.</math>

Clearly we can assign meaning to any such power series of quotients (orbits) with respect to permutation groups, where we restrict the groups of degree ''n'' to the conjugacy classes <math>\operatorname{Cl}(S_n)</math> of the symmetric group <math>S_n</math>, which form a [[unique factorization ___domain]]. (The orbits with respect to two groups from the same conjugacy class are isomorphic.) This motivates the following definition.

A class <math>\mathcal{C}\in \mathbb{N}[\mathfrak{AM}]</math> of combinatorial structures is a formal series

:<math>\mathcal{C} = \sum_{n \ge 1} \sum_{G\in \operatorname{Cl}(S_n)} c_G (X^n/G)</math>

where <math>\mathfrak{AM}</math> (the "AM" is for "atomsmolecules") is the set of primes of the UFD <math>\{\operatorname{Cl}(S_n)\}_{n\ge 1}</math> and <math>c_G \in \mathbb{N}.</math>

In the following we will simplify our notation a bit and write e.g.

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This operator corresponds to the class

:<math>L = \frac{1}{1 - X} = 1 + E_1X + E_2X^2 + E_3X^3 + \cdots</math>

and represents sequences, i.e. the slots are not being permuted and there is exactly one empty sequence. We have

:<math> F(z) = 1 + \sum_{n\ge 1} Z(E_n1)(f(z), f(z^2), \ldots, f(z^n)) =

1 + \sum_{n\ge 1} f(z)^n = \frac{1}{1-f(z)}</math>

and

:<math> G(z) = 1 + \sum_{n\ge 1} \left(\frac{1}{|E_n|}\right) g(z)^n =

\frac{1}{1-g(z)}.</math>

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This operator corresponds to the class

:<math>C = C_1 + C_2 + C_3 + \cdots</math>

i.e., cycles containing at least one object. We have

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The series is

:<math>E = 1 + S_1E_1 + S_2E_2 + S_3E_3 + \cdots</math>

i.e., the symmetric group <math>S_n = E_n</math> is applied to the slots''n''th slot. This creates multisets in the unlabelled case and sets in the labelled case (there are no multisets in the labelled case because the labels distinguish multiple instances of the same object from the set being put into different slots). We include the empty set in both the labelled and the unlabelled case.

The unlabelled case is done using the function

:<math>M(f(z), y) = \sum_{n\ge 0} y^n Z(S_nE_n)(f(z), f(z^2), \ldots, f(z^n))</math>

so that

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:<math>\sum_{k=0}^n A_k B_{n-k}.</math>

Using the definition of the OGF and some [[elementary algebra]], we can show that

:<math>\mathcal{A} = \mathcal{B} \times \mathcal{C}</math> implies <math>A(z) = B(z) \cdot C(z).</math>