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Find the triples

Source: INMO 1996 Problem 4

October 6, 2005

combinatorics unsolvedcombinatorics

Problem Statement

Let

X

be a set containing

n

elements. Find the number of ordered triples

(A,B, C)

of subsets of

X

such that

A

is a subset of

B

and

B

is a proper subset of

C

.

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