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Reference.org
Cannonball problem
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<p>In the mathematics of <a href="/facts/Figurate_number/f1gdGcja">figurate numbers</a>, the cannonball problem asks which numbers are both <a href="/facts/Square_number/jfakASPK">square</a> and <a href="/facts/Square_pyramidal_number/3WbRdj34">square pyramidal</a>. The problem can be stated as: given a square arrangement of cannonballs, for what size squares can these cannonballs also be arranged into a square pyramid. Equivalently, which squares can be represented as the sum of consecutive squares, starting from 1. </p>