Diferente pentru blog/combinatorics-shortlist intre reviziile #23 si #22

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How many stars will be printed for a given n and k.
 
#8 x
# 9y
 
#8 [1] How many ways can k rooks be placed on a nxn chessboard so that they don’t attack each other.
# (romanian county olympiad, 11th grade 2000 [1]) How many permutations of length n have no fixed points. A fixed point in a permutation p is p(i) = i
# (county level olympiad, 1994 [1]) How many correct bracket sequences of length 2n are there.

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