Insight · ★★★☆☆ · ~7 min
Square Infection (Perimeter Invariant)
Infection spreads to cells with ≥2 sick neighbors. Can k cells fill an n×n grid?
by MindYourPuzzle
On an n×n grid, cells can be infected. A healthy cell becomes infected if at least 2 of its 4 neighbors are infected.
Prove you cannot infect the entire board starting from fewer than n infected cells.
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