`R/SplitFunctions.R`

`NPartitionPairs.Rd`

`NPartitionPairs()`

calculates the number of terminal arrangements matching
a specified configuration of two splits.

`NPartitionPairs(configuration)`

- configuration
Integer vector of length four specifying the number of terminals that occur in both (1) splits A1 and A2; (2) splits A1 and B2; (3) splits B1 and A2; (4) splits B1 and B2.

The number of ways to distribute `sum(configuration)`

taxa according
to the specified pattern.

Consider splits that divide eight terminals, labelled A to H.

Bipartition 1: | ABCD:EFGH | A1 = ABCD | B1 = EFGH |

Bipartition 2: | ABE:CDFGH | A2 = ABE | B2 = CDFGH |

This can be represented by an association matrix:

A2 | B2 | |

A1 | AB | C |

B1 | E | FGH |

The cells in this matrix contain 2, 1, 1 and 3 terminals respectively; this
four-element vector (`c(2, 1, 1, 3)`

) is the `configuration`

implied by
this pair of bipartition splits.

```
NPartitionPairs(c(2, 1, 1, 3))
#> [1] 12
```