ISI 2017

[Prakash Kapadia]

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

[Saumik Karfa]

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

[Author]

Posts