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Square triangular number

In mathematics, a square triangular number (or triangular square number) is a number which is both a triangular number and a square number, in other words, the sum of all integers from 1 {\displaystyle 1} to n {\displaystyle n} has a square root that is an integer. There are infinitely many square triangular numbers; the first few are:

Characters, Other characterizations & Solution as a Pell equation