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Coloring of a board by two colours

Source: IMOTC 2015 Practice Test 2 Problem 3

July 11, 2015
combinatorics

Problem Statement

Every cell of a 3×33\times 3 board is coloured either by red or blue. Find the number of all colorings in which there are no 2×22\times 2 squares in which all cells are red.
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