A380825 Indices of triangular numbers that are the products of triangular numbers larger than 1.

8, 9, 20, 24, 27, 35, 39, 44, 54, 55, 75, 80, 84, 90, 98, 99, 104, 132, 135, 153, 175, 189, 195, 207, 224, 230, 231, 252, 260, 272, 275, 279, 285, 296, 324, 350, 351, 374, 399, 405, 440, 455, 459, 475, 494, 539, 560, 567, 575, 594, 615, 620, 624, 665, 675

Offset: 1

Author

Kelvin Voskuijl, Feb 04 2025

Keywords

• nonn

Examples

			20 is a term as the 20th triangular number is 210 = 21*10 both triangular numbers.
24 is also a term since the 24th triangular number is 300 = 3*10*10.
		

Crossrefs

Cf. A000217, A003056, A068143.

Formula

a(n) = A003056(A068143(n)).

A000217(a(n)) = A068143(n).