ISI 2017

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  • #70866
    Prakash Kapadia
    Participant

    Let A = {1,2,…,n}. For a permutation P = ( P(1),P(2),…,P(n) ) of all elements of A , let P(1) denote the first element of P. Find the first element of P . Find the number of all such permutations P so that for all i,j ∈ A:

    • if i<j<= P(1), then j appears before i in P; and
    • if P(1)<i<j , then i appears before j in P
    #70919
    Saumik Karfa
    Moderator

    This problem is discussed in AOPS. We do not wish to rewrite the same thing. Here is the direct link that contains the discussion :

    https://artofproblemsolving.com/community/c6h1446840p8270148

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