The squares of triangular numbers 1 and 6 are triangular numbers 1 and 36.
T1^2 = 1 * 1 = 1 = T1
T3^2 = 6 * 6 = 36 = T8
Are there additional triangular numbers whose squares form a triangular number?
After posting the "official" solution I would like to add, just for the record: all of us somehow missed 0 (zero), a number qualifying both as a triangular number and as a square.