MathDB
2012-2013 Winter OMO #24

Source:

January 16, 2013
Online Math Open

Problem Statement

For a permutation π\pi of the integers from 1 to 10, define S(π)=i=19(π(i)π(i+1))(4+π(i)+π(i+1)), S(\pi) = \sum_{i=1}^{9} (\pi(i) - \pi(i+1))\cdot (4 + \pi(i) + \pi(i+1)), where π(i)\pi (i) denotes the iith element of the permutation. Suppose that MM is the maximum possible value of S(π)S(\pi) over all permutations π\pi of the integers from 1 to 10. Determine the number of permutations π\pi for which S(π)=MS(\pi) = M.
Ray Li