
COMMENTS

Ljunggren used padic methods to prove that 0, 1, and 36 are the only triangular numbers that are squares of triangular numbers.
Same as: the only positive integer solutions of (x(x1))^2 = 2y(y1) are (x,y) = (1,1), (2,2), and (4,9).
Cassels used elliptic curves to simplify Ljunggren's proof.
Subsequence of A001110 (triangular numbers that are squares).
