Re: [math-fun] triangular sums

It is sequence A051533 but no characterization is included. Emeric On Sat, 18 Dec 2004, William Thurston wrote:

There's a nice, well-known theory of which integers can be expressed as the sum of two squares. What can be said about numbers that are sums of two triangles, i.e. 1 or 3 or 6 or 10 or 15 ... + 1 or 3 or 6 or 10 or 15 ...?

Bill

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