UGC-NET | UGC NET CS 2016 July – III | Question 32
Let A[1…n] be an array of n distinct numbers. If i < j and A[i] > A[j], then the pair (i, j) is called an inversion of A. What is the expected number of inversions in any permutation on n elements ?
There are n(n-1)/2 pairs such that i < j. For a pair (ai, aj), probability of being inversion is 1/2. Therefore expected value of inversions = 1/2 * (n(n-1)/2) = n(n-1)/4.
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