## Nontransitive sets of dice

### Problem 376

Published on Sunday, 18th March 2012, 01:00 am; Solved by 185; Difficulty rating: 70%Consider the following set of dice with nonstandard pips:

Die A: 1 4 4 4 4 4

Die B: 2 2 2 5 5 5

Die C: 3 3 3 3 3 6

A game is played by two players picking a die in turn and rolling it. The player who rolls the highest value wins.

If the first player picks die A and the second player picks die B we get

P(second player wins) = ^{7}/_{12} > ^{1}/_{2}

If the first player picks die B and the second player picks die C we get

P(second player wins) = ^{7}/_{12} > ^{1}/_{2}

If the first player picks die C and the second player picks die A we get

P(second player wins) = ^{25}/_{36} > ^{1}/_{2}

So whatever die the first player picks, the second player can pick another die and have a larger than 50% chance of winning.

A set of dice having this property is called a **nontransitive set of dice**.

We wish to investigate how many sets of nontransitive dice exist. We will assume the following conditions:

- There are three six-sided dice with each side having between 1 and
`N`pips, inclusive. - Dice with the same set of pips are equal, regardless of which side on the die the pips are located.
- The same pip value may appear on multiple dice; if both players roll the same value neither player wins.
- The sets of dice {A,B,C}, {B,C,A} and {C,A,B} are the same set.

For `N` = 7 we find there are 9780 such sets.

How many are there for `N` = 30 ?