# 666

This number is a composite.

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666 = (3*5)^{2}+ (3*7)

^{2}, written with all the single-digit primes. Note that 666, which is formed from the triangular number 6 repeated 3 (the only prime triangular number) times, is the smallest triangular number n = a

^{2}+ b

^{2}such that a, b and a+b are triangular numbers. [Capelle]

Submitted: 2009-05-09 05:50:47; Last Modified: 2009-05-09 10:31:52.

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