# nLab order polynomial

### Context

#### Combinatorics

combinatorics

enumerative combinatorics

graph theory

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### Polytopes

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category: combinatorics

# Contents

## Idea

The order polynomial $\Omega_P(n)$ of a finite partially ordered set $P$ counts the number of ways that $P$ can be extended to a linear order of size $n$.

## Definition

By a linear extension of $P$ of size $n$, we mean an order-preserving function from $P$ to $(n) = \{ 1 \lt \cdots \lt n \}$. To see that

$\Omega_P(n) = |Hom(P,(n))|$

defines a polynomial in $n$, first observe that any function $P \to (n)$ factors as a surjection from $P$ onto some $(k) = \{ 1 \lt \cdots \lt k \}$ (where $k \le n$), followed by an injection from $(k)$ to $(n)$. The total number of order-preserving functions from $P$ to $(n)$ can therefore be calculated explicitly as

$\Omega_P(n) = \sum_{k=1}^{|P|} e_k \binom{n}{k}$

where $e_k$ is the number of surjective order-preserving functions from $P$ to $(k)$. Hence $\Omega_P(n)$ is a polynomial of degree equal to the cardinality of $P$.

## Examples

The order polynomial of $(3) = \{ 1 \lt 2 \lt 3 \}$ is

$n + 2 \binom{n}{2} + \binom{n}{3} = \frac{n^3 + 3n^2 + 2n}{6}$

For example, evaluating the polynomial at $n=3$ confirms that there are 10 order-preserving functions from $(3)$ to itself.

The order polynomial of the 3-element poset

$P = \array{&&c&& \\ &\nearrow& &\nwarrow& \\ a &&&& b}$

is $n + 3 \binom{n}{2} + 2\binom{n}{3} = \frac{2n^3 + 3n^2 + n}{6}$. Evaluating at $n=2$, we compute that there are 5 order-preserving functions from $P$ onto $(2)$.

## Properties

### Relation to zeta polynomial

Let $P^\downarrow \cong (2)^P$ denote the lattice of lower sets in $P$. The order polynomial of a finite poset $P$ is related to the zeta polynomial of its lattice of lower sets $P^\downarrow$ by the equation1

$\Omega_P(n+2) = Z_{P^\downarrow}(n)$

This can be seen as a consequence of the currying isomorphisms

$Hom([n], P^\downarrow) \cong Hom([n] \times P, (2)) \cong Hom(P, [n]^\downarrow)$

together with the isomorphisms $[n]^\downarrow \cong [n+1] \cong (n+2)$.

### Relation to the strict order polynomial

Let $\bar{\Omega}_P(n)$ denote the number of strict order-preserving functions from $P$ to $(n)$, that is, functions $f : P \to (n)$ such that $x \lt y$ implies $f(x) \lt f(y)$. This again defines a polynomial in $n$, called the strict order polynomial of $P$. The strict order polynomial of a finite poset $P$ is related to its order polynomial by the equation

$\bar{\Omega}_P(n) = (-1)^p \Omega_P(-n)$

where $p = |P|$ is the cardinality of $P$. This is an example of a combinatorial reciprocity theorem in the sense of Stanley (1974).

(Notice that we are evaluating the order polynomial at a negative integer, even though the putative definition $\Omega_P(n) = |Hom(P,(n))|$ only makes sense for natural numbers $n$. This is justified because the value of a polynomial on the natural numbers determines its value on arbitrary integers.)

As an example, the strict order polynomial of the 3-element poset $P$ defined above is $\bar{\Omega}_P(n) = \frac{2n^3 - 3n^2 + n}{6}$. Evaluation at $n=2$ confirms that there is exactly one strict monotone map from $P$ to a 2-element linear order (sending both $a$ and $b$ to the first element and $c$ to the second element).

Through Möbius inversion, this equation relating the strict order polynomial to the order polynomial can be connected to the previous one relating the order polynomial to the zeta polynomial. Since the lattice of lower sets $P^\downarrow$ has bottom and top elements, we know that

$\Omega_P(n) = Z_{P^\downarrow}(n-2) = \zeta^n(\emptyset,P)$

where $\zeta^n$ is the $n$-fold convolution of the zeta function of $P^\downarrow$. One can similarly work out that

$\bar{\Omega}_P(n) = (-1)^p\mu^n(\emptyset,P)$

where $\mu^n$ is the $n$-fold convolution of the Möbius function of $P^\downarrow$. The equation $\bar{\Omega}_P(n) = (-1)^p \Omega_P(-n)$ then follows formally from the fact that $\mu = \zeta^{-1}$.

### Computational complexity

Counting the number of total linear extensions of a poset (i.e., number of linear extensions of an $n$-element poset to a linear order of size $n$) is a #P-complete problem (Brightwell and Winkler 1991), and thus so is the problem of computing the leading coefficient of the order polynomial (cf. Browning, Hopkins, and Kelley 2017).

• Richard P. Stanley. Ordered structures and partitions. Memoirs of the AMS 119, 1972. (doi) (Free copy available from the author’s website, see #9.)
• Richard P. Stanley. Combinatorial Reciprocity Theorems. Advances in Mathematics 14:194-253, 1974. (Free copy available from the author’s website, see #22.)
• Joseph P. S. Kung, Gian-Carlo Rota, Catherine H. Yan. Combinatorics: The Rota Way. Cambridge, 2009.

• Graham Brightwell and Peter Winkler. Counting Linear Extensions. Order 8:225-242, 1991. doi

• Thomas Browning, Max Hopkins, and Zander Kelley. Doppelgangers: the Ur-Operation and Posets of Bounded Height. arXiv:1710.140407

category: combinatorics

1. Note that the definition we use for $Z_P(n)$ has an index shift from the definition that seems to be more standard in combinatorics (see the footnote at zeta polynomial), so this equation is sometimes expressed as $\Omega_P(n) = Z_{P^\downarrow}(n)$.