MathDB
Polish Combinatorics

Source: Poland - Second Round P2

February 8, 2020
combinatoricscomputer science

Problem Statement

Let nn be a positive integer. Jadzia has to write all integers from 11 to 2n12n-1 on a board, and she writes each integer in blue or red color. We say that pair of numbers i,j{1,2,3,...,2n1}i,j\in \{1,2,3,...,2n-1\}, where iji\leqslant j, is <spanclass=latexitalic>good</span><span class='latex-italic'>good</span> if and only if number of blue numbers among i,i+1,...,ji,i+1,...,j is odd. Determine, in terms of nn, maximal number of good pairs.