Fermat’s polygonal number theorem

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Note that Lagrange’s four-square theorem is a special case of Fermat’s polygonal number theorem.
keywords:
[Mathematics]
[Number Theory]
[b^2 < 4a]
[3a < b^2 + 2b + 4]
see also:
[‘regular’ polygonal numbers]
[n-gonal numbers]
[additive basis of order n]