Triangular Squares

The numbers that are both square and triangular are beautifully related to the best approximations to √2:


Square Root

Factors of root

1 1 1
36 6 2 * 3
1225 35 5 * 7
41616 204 12 * 17

and so on.

In each case the factors of the root are the numerator and denominator of the next approximation to √2. — David Wells, The Penguin Dictionary of Curious and Interesting Mathematics (1986), entry for “36”.

Elsewhere other-accessible

A001110 — Square triangular numbers: numbers that are both triangular and square

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