# Contents

## Idea

The term ‘shuffle’ conjures up the idea of shuffling a pack of cards. In fact the mathematical idea is nearer to shuffling two packs of cards one through the other. Suppose we have a pack of $p$ cards and a pack of $q$ cards and we build a pack of $p+q$ cards, whilst retaining the order on the two ‘sub-packs’. The result is a $(p,q)$-shuffle.

## Definition

### Shuffles

###### Definition

For $p,q \in \mathbb{N}$ two natural numbers, a **$(p,q)$-shuffle** is a permutation

$(\mu_1, \cdots, \mu_p, \nu_1, \cdots, \nu_q)$

of $(1,2, \cdots, p+q)$ subject to the condition that

$\mu_1 \lt \mu_2 \lt \cdots \lt \mu_p$

and

$\nu_1 \lt \nu_2 \lt \cdots \lt \nu_q
\,.$

The **signature** of a $(p,q)$-shuffle is the signature of the corresponding permutation.

### Unshuffles

The same concept viewed from the other end leads to *unshuffles* . These are just shuffles but are used in dual situations in the applications. The definition that follows is ‘from the literature’. It is equivalent to that of shuffle that we gave above. (Although not needed, it is important to note the different terminology used in certain applications of the idea for when original source material is consulted.)

###### Definition

We say that a permutation $\sigma\in S_n$ is a $(j,n-j)$-unshuffle, $o\leq j\leq n$ if $\sigma(1)\lt \ldots \sigma(j)$ and $\sigma(j+1)\lt \ldots \lt \sigma(n)$.

You can also say that $\sigma$ is a $(j,n-j)$-unshuffle if $\sigma(i) \lt \sigma(i+1)$ when $i\neq j$.

## Applications

### Products of simplices

Shuffles control the combinatorics of products of simplices. See products of simplices for details.

### Eilenberg-Zilber map

Related to the product of simplices: shuffles control the Eilenberg-Zilber map. See there for details.

### Differential graded coalgebras

Shuffles are used in defining the pre-cgc structure on $\bigwedge V$ in the theory of differential graded coalgebras

### Differential graded Hopf algebras

Shuffles are also used for defining the shuffle product on $T(V)$, see differential graded Hopf algebra.

### In $L_\infty$-algebras

In the definition of L-∞ algebras the *unshuffle* side of shuffles is used.