# Puzzle in today_s Guardian_Certain sq

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."

http://www.conwaylife.com/forums/viewtopic.php?f=11&t=814

"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."

http://www.conwaylife.com/forums/viewtopic.php?f=11&t=814

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