MathDB

We are given an infinite deck of cards, each with a real number on it. For every real number

x

, there is exactly one card in the deck that has

x

written on it. Now two players draw disjoint sets

A

and

B

100

cards each from this deck. We would like to define a rule that declares one of them a winner. This rule should satisfy the following conditions: 1. The winner only depends on the relative order of the

200

cards: if the cards are laid down in increasing order face down and we are told which card belongs to which player, but not what numbers are written on them, we can still decide the winner. 2. If we write the elements of both sets in increasing order as

A =\{ a_1 , a_2 , \ldots, a_{100} \}

and

B= \{ b_1 , b_2 , \ldots , b_{100} \}

, and

a_i > b_i

for all

i

, then

A

beats

B

. 3. If three players draw three disjoint sets

A, B, C

from the deck,

A

beats

B

and

B

beats

C

then

A

also beats

C

. How many ways are there to define such a rule? Here, we consider two rules as different if there exist two sets

A

and

B

such that

A

beats

B

according to one rule, but

B

beats

A

according to the other.
Proposed by Ilya Bogdanov, Russia

Problem Statement