What numbers can be written as the sum of two triangular numbers? i.e.,
$$\frac{x(x+1)}{2}+\frac{y(y+1)}{2}=z $$
$4z+1$ is either the sum of two squares or itself a square. Why?
2026-03-30 04:53:30.1774846410
Sum of Two Triangular Numbers?
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