MathDB
2015-2016 Spring OMO #16

Source:

March 29, 2016
Online Math Open

Problem Statement

Jay is given a permutation {p1,p2,,p8}\{p_1, p_2,\ldots, p_8\} of {1,2,,8}\{1, 2,\ldots, 8\}. He may take two dividers and split the permutation into three non-empty sets, and he concatenates each set into a single integer. In other words, if Jay chooses a,ba,b with 1a<b<81\le a< b< 8, he will get the three integers p1p2pa\overline{p_1p_2\ldots p_a}, pa+1pa+2pb\overline{p_{a+1}p_{a+2}\ldots p_{b}}, and pb+1pb+2p8\overline{p_{b+1}p_{b+2}\ldots p_8}. Jay then sums the three integers into a sum N=p1p2pa+pa+1pa+2pb+pb+1pb+2p8N=\overline{p_1p_2\ldots p_a}+\overline{p_{a+1}p_{a+2}\ldots p_b}+\overline{p_{b+1}p_{b+2}\ldots p_8}. Find the smallest positive integer MM such that no matter what permutation Jay is given, he may choose two dividers such that NMN\le M.
Proposed by James Lin