### IMO Shortlist 2010 problem N6

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June 23, 2013
The rows and columns of a $2^n \times 2^n$ table are numbered from $0$ to $2^{n}-1.$ The cells of the table have been coloured with the following property being satisfied: for each $0 \leq i,j \leq 2^n - 1,$ the $j$-th cell in the $i$-th row and the $(i+j)$-th cell in the $j$-th row have the same colour. (The indices of the cells in a row are considered modulo $2^n$.) Prove that the maximal possible number of colours is $2^n$.

Proposed by Hossein Dabirian, Sepehr Ghazi-nezami, Iran
Source: Međunarodna matematička olimpijada, shortlist 2010