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P23 [Combinatorics] - Turkish NMO 1st Round - 2004

Source:

November 15, 2013

Problem Statement

What is the maximal possible value of nn such that no matter how 2525 squares are selected in an infinite chessboard one can find nn squares in which none of them share a common corner?
<spanclass=latexbold>(A)</span> 7<spanclass=latexbold>(B)</span> 8<spanclass=latexbold>(C)</span> 9<spanclass=latexbold>(D)</span> 10<spanclass=latexbold>(E)</span> 11 <span class='latex-bold'>(A)</span>\ 7 \qquad<span class='latex-bold'>(B)</span>\ 8 \qquad<span class='latex-bold'>(C)</span>\ 9 \qquad<span class='latex-bold'>(D)</span>\ 10 \qquad<span class='latex-bold'>(E)</span>\ 11