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m x 1 and 1 x m rectangles in an nxn board, equal no of type I,II, min N

Source: Dutch IMO TST 2013 day 1 p4

August 29, 2019
combinatoricscombinatorial geometryrectangleboard

Problem Statement

Let n3n \ge 3 be an integer, and consider a n×nn \times n-board, divided into n2n^2 unit squares. For all m1m \ge 1, arbitrarily many 1×m1\times m-rectangles (type I) and arbitrarily many m×1m\times 1-rectangles (type II) are available. We cover the board with NN such rectangles, without overlaps, and such that every rectangle lies entirely inside the board. We require that the number of type I rectangles used is equal to the number of type II rectangles used.(Note that a 1×11 \times 1-rectangle has both types.) What is the minimal value of NN for which this is possible?