category theory

# Contents

## Idea

A (combinatorial) species is a presheaf or higher categorical presheaf on the groupoid core(FinSet).

A species is a symmetric sequence by another name. Meaning: they are categorically equivalent notions.

## Definition

### 1-categorical

The category of species, $Species := PSh(core(FinSet))$, is the category of presheaves on the groupoid $\mathbb{P} := core(FinSet)$ of finite sets and bijections, the core of FinSet:

$\hat \mathbb{P} := PSh(core(FinSet)) \,.$

For more, see structure type.

### 2-categorical

A 2-species (usually called a stuff type) is a 2-presheaf on core(FinSet), i.e. a pseudofunctor

$core(FinSet) \to Grpd$

to the 2-category Grpd.

### $(\infty,1)$-categorical

Generally, an $\infty$-species or homotopical species is an (∞,1)-presheaf on $core(FinSet)$, i.e. an (∞,1)-functor

$core(FinSet)^{op} \to \infty Grpd$

to the (∞,1)-category ∞Grpd.

The (∞,1)-category of $\infty$-species is the (∞,1)-category of (∞,1)-presheaves

$\infty Species := PSh_{(\infty,1)}(core(FinSet)) \,,$

For more on this see also below the discussion In homotopy type theory.

### Operations on species

There are in fact 5 important monoidal structures on the category of species. For a discussion of all five, you’ll currently have to read about Schur functors, where these operations are discussed in the context of $Fin\Vect$-valued species, i.e. $Fin\Vect$-valued presheaves on the groupoid of finite sets. But here are two:

#### Sum

The sum $A + B$ of two species $A$, $B$ is their coproduct $A \coprod B$. Since colimits of presheaves are computed objectwise, we have

$(A + B)_n \simeq A_n \coprod B_n \,.$

#### Product

The category $core(FinSet)$ becomes a monoidal category under disjoint union of finite sets. This monoidal structure $(core(FinSet), \coprod)$ induces canonically the Day convolution monoidal structure on $Species := PSh(core(FinSet))$.

For $A$ and $B$ two combinatorial species, their product is given by the coend

$(A \otimes B)_n \simeq \int^{n \in FinSet} A_k \times B_l \times Hom_{core(FinSet)}(n,k+l) \simeq \coprod_{k+l = n} \prod_{\frac{(k+l)!}{k! + l!}} (A_k \times B_l) \,.$

## In Homotopy Type Theory

The following discusses formalization of the concept of species in homotopy type theory.

Let FinSet be the type of finite sets (see at hSet). Notice that in homotopy type theory this automatically comes out as the groupoid of finite sets (see also at universe in type theory), so we may (and do) suppress writing “core” here.

###### Definition

A species is a type $X \colon Type$ equipped with a function into $FinSet$. Hence the type of all species is the dependent sum

$Species \coloneqq \sum_{X : Type} (X \to FinSet)$

of all function types $X \to FinSet$ as $X$ ranges over the type of types.

Write

$FinSet \longrightarrow {\|FinSet\|_{0}} \simeq \mathbb{N}$

for the unit of the 0-truncation modality on FinSet, going into the type of natural numbers.

Given a species $f \colon X \to FinSet$ as in def. 1, its “decategorification” is the composite

$f \circ card \colon X \stackrel{f}{\longrightarrow} FinSet \to {\|FinSet\|_{0}} \simeq \mathbb{N}$
###### Definition

The generating function of a species $f \colon X \to FinSet$, def. 1, is

${|X|}(z) \coloneqq \sum_{n=0}^{\infty} {\big| fib_{f \circ card}(n) \big|}\, z^{n} = \sum_{n=0}^{\infty} {\big| fib_{f}(Fin(n)) \big|}\, \frac{z^{n}}{n!} \,,$

where $fib(-)$ denotes the homotopy fiber and $Fin(n)$ denotes a 0-type with exactly $n$ elements.

###### Definition

If $\Phi : FinSet \to Type$ is a family of types, then we obtain the species

$pr_{1} : \bigg(\sum_{A : FinSet} \Phi(A)\bigg) \to FinSet$

with generating function

${|\Phi|}(z) = \sum_{n=0}^{\infty} {|\Phi(Fin(n))|} \frac{z^{n}}{n!}$
###### Example

If $\Phi$ is a stuff, structure, or property on finite sets, then we immediately know something about its generating function, considered as an exponential generating function:

• If $\Phi(A)$ is contractible for all $A$, then $\Phi(Fin(n)) = 1$, so its generating function is the exponential function $e^{z}$.

• If $\Phi(A)$ is a family of mere propositions, then every coefficient is either $0$ or $1$, according to whether $A$ can be equipped with a $\Phi$-structure or not.

• If $\Phi(A)$ is a family of sets (i.e., a structure type), then every coefficient is an element of the type of natural numbers $\mathbb{N}$.

• If $\Phi(A)$ has a higher homotopy level (i.e., is a stuff type), then the coefficients are in $[0, \infty)$.

###### Remark

Since any species $f : X \to Type$ is equal to

$pr_{1} : \bigg(\sum_{x:X}fib_{f}(x)\bigg) \to Type$

every species is equal to one obtained from a stuff type.

### Operations on species

The five monoidal structures mentioned above under Operations on species can be represented in HoTT, along with a couple other useful operations. We give four of the five monoidal structures here. For more operations, see (Dougherty15). We assume throughout this subsection that $f : X \to \Finset$ and $g : Y \to Finset$ are two species, and $\Phi, \Psi : FinSet \to Type$ are two stuff types.

#### Coproduct

The recursion principle for the coproduct gives the species

$\array{ (f + g) &:& X + Y \to Finset \\ (f + g) &:& rec_{X + Y}(FinSet, f, g) = z \mapsto \left\{\array{ f(x) & \text{if}\; z \equiv inl x \\ g(y) & \text {if}\; z \equiv inr y .}\right. }$

If $X$ and $Y$ are obtained from $\Phi$ and $\Psi$, respectively, then the coproduct is equal to

$\sum_{A : FinSet} (\Phi(A) + \Psi(A))$

The generating function for the coproduct is

${|X + Y|}(z) = {|X|}(z) + {|Y|}(z)$

The Hadamard product of species is

$\array{ \langle X, Y \rangle &:& X \times_{FinSet} Y \to FinSet \\ \langle X, Y \rangle &:& (x, y, p) \mapsto f(x) }$

which for stuff types is equal to

$\array{ \sum_{A : FinSet} (\Phi(A) \times \Psi(A)) }$

The generating functions are related by

${|\langle X, Y \rangle|}(z) = \sum_{n=0}^{\infty} n! X_{n} Y_{n} z^{n}$

where $X_{n}$ and $Y_{n}$ are the $n$th coefficients of the functions ${|X|}(z)$ and ${|Y|}(z)$, respectively.

The name “Hadamard product” is used in (Aguiar-Mahajan10) and (Bergeron-Labelle-Leroux08). In (Baez-Dolan01) it is called the “inner product” of stuff types, because equipping the category of stuff types with this operation makes it a categorified version of the Hilbert space of a quantum harmonic oscillator.

#### Cauchy product

###### Definition

The Cauchy product of species is given by sending two species $f,g$ as in def. 1 to the species given by

$\array{ (f \cdot g) &:& X \times Y \to FinSet \\ (f \cdot g) &:& (x, y) \mapsto f(x) + g(y) }$
###### Remark

The Cauchy product, def. 4, is eqivalently the phased tensor product on the slice over the symmetric monoidal groupoid $(FinSet, \coprod)$.

###### Example

For stuff types $\Phi$, $\Psi$, their Cauchy product, def. 4, is equal to

$\sum_{A, U, V : FinSet} \sum_{p : U + V = A} (\Phi(U) \times \Psi(V))$

In this form the Cauchy product is discussed in (Aguiar-Mahajan 10, def. 8.5, Aguiar-Mahajan 12, section 2.2). Monoids and comonoid in the resulting monoidal category of species are discussed in (Aguiar-Mahajan 10, section 8.2) and Hopf monoids in this category in (Aguiar-Mahajan 12, section 2.2).

This means that $\Phi\Psi$ stuff on a finite set is the stuff of “being chopped in two, with $\Phi$ stuff on one part and $\Psi$ stuff on the other”.

The generating function is

${|X \cdot Y|}(z) = {|X|}(z) \cdot {|Y|}(z) \,.$

#### Composition

The composition of two species is

$\array{ (f \tilde{\circ} g) &:& \bigg(\sum_{x:X}(Fin(card(f(x)) \to Y)\bigg) \to FinSet \\ (f \tilde{\circ} g) &:& (x, P, p) \mapsto \bigoplus_{i=1}^{card(f(x))}g(P(i)) }$

where $\bigoplus_{i=1}^{n}g(P(i))$ is the $n$-ary direct sum of the finite sets $g(P(i))$. For stuff types, this amounts to

$\sum_{(A, C : FinSet)}\sum_{(B : Fin(card(C)) \to FinSet)} \bigg( (B\vdash_{card(C)}A) \times \Phi(C) \times \prod_{k : Fin(card(C))} \Psi(B(k)) \bigg)$

That is, $\Phi \tilde{\circ} \Psi$ stuff on a finite set is a partition of the set into $card(C)$ subsets, with $\Phi$ stuff on the partition and $\Psi$ stuff on each of the subsets.

The generating function of the composition is

${|X \tilde{\circ} Y|}(z) = {|X|}({|Y|}(z))$

This species is also called the plethysm product.

## Properties

### Cardinality

Under groupoid cardinality

${|-|} : \infty Grpd_{tame} \to \mathbb{R}$

every (tame) ∞-groupoid is mapped to a real number

$X \mapsto {|X|} := \sum_{[x] \in \pi_0(X)}\prod_{i = 1}^{\infty} (\pi_i(X,x)^{(-1)^{i}}) \,.$

A species $\mathbf{X}$ assigns an ∞-groupoid $\mathbf{X}_n$ to each natural number $n \in \mathbb{Z}$. Therefore under groupoid cardinality we may naturally think of a tame species as mapping to a power series

$\mathbf{X} \mapsto {|\mathbf{X}|} := \sum_{n = 0}^{\infty} \frac{1}{n!} {|\mathbf{X}_n|} z^n \in \mathbb{R}[ [ z ] ] \,.$

This cardinality operation maps the above addition and multiplication of combinatorial species to addition and multiplication of power series.

That coproduct of species maps to sum of their cardinalities is trivial. That Day convolution of species maps under cardinality to the product of their cardinality series depends a little bit more subtly on the combinatorial prefactors:

${| A \otimes B |} = {|A|} \cdot {|B|} = \sum_{n=0}^\infty \frac{1}{n!} \sum_{k+l = n} \frac{n!}{k! l!} {|A_k|} \cdot {|B_l|} \,.$

## Variants

• If in the definition of combinatorial species the domain core(FinSet) is replaced with FinVect and also the presheaves are take with values in FinVect then one obtains the notion of Schur functor.

• Instead of $core(FinSet)$, we may choose as domain $\mathbf{B} A$ for a small category $A$, with objects $\langle a_i \rangle_{i \in I}$, for $I$ in $core(FinSet)$, i.e., finite families of objects of $A$. Also, instead of $Set$ valued presheaves, we may consider those valued in presheaves on a small category $B$. This joint generalisation yields what are called generalised species. These generalised species can be collected in a cartesian closed 2-category (FGHW).

## References

The notion of species goes back to

An expositional discussion can be found at

• Todd Trimble, Exponential Generating Function and Introduction to Species (blog) (scroll down a bit).

An application in computer science:

• Brent Yorgey, Random testing and beyond! with combinatorial species, slides pdf; Species and functors and types, oh my, article, pdf

Formalization in homotopy type theory:

An application in statistical mechanics:

• W. Faris, Combinatorial species and cluster expansions, Mosc. Math. J. 10:4 (2010), 713–727 pdf, MR2791054

Discussion in relation to Feynman diagrams:

• John Baez and James Dolan, From finite sets to Feynman diagrams, in Mathematics Unlimited - 2001 and Beyond, vol. 1, eds. Björn Engquist and Wilfried Schmid, Springer, Berlin, 2001, pp. 29-50. (arXiv)

Discussion of generalised species:

• M. Fiore, N. Gambino, M. Hyland, G. Winskel, The cartesian closed bicategory of generalised species of structures, (pdf)

Revised on July 25, 2016 03:35:05 by Urs Schreiber (89.15.236.92)