Puzzle in today_s Guardian_Certain sq

Puzzle in today&#39;s Guardian:<br>&quot;Certain squares in an n x n array gr...
Tim HuttonTim Hutton - 2012-01-14 23:25:25+0000 - Updated: 2012-01-14 23:25:25+0000
Puzzle in today's Guardian:
"Certain squares in an n x n array grid have become infected. The rules of transmission are simple. When two or more edges of a square become infected squares it also becomes infected. Show that for an n x n array of squares at least n squares must be infected for all to become so. The example shows a 4 x 4 array with 4 squares infected."
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