A274686 Least number k such that k-th triangular number is the sum of two nonzero squares in exactly n ways.

4, 40, 25, 145, 625, 169, 31249, 985, 2600, 2500, 87890625, 3649, 384199200, 15625, 33124, 6409

Offset: 1

Author

Altug Alkan, Jul 02 2016

Keywords

• nonn
• more

Comments

From Robert Israel, Jul 04 2016: (Start)

Least k such that A025426(A000217(k)) = n.

A025426(A000217(18463134765625))=17, but I don't know if this is minimal. (End)

a(18) = 24649, a(20) = 40000, a(21) = 250000. 25*10^6, 25*10^8, 25*10^12 are not terms. Are there other terms of the form 25*10^(2k)? - Chai Wah Wu, Jul 23 2020

Examples

			a(2) = 40 because 40*41 / 2 = 820 = 6^2 + 28^2 = 12^2 + 26^2.
		

Crossrefs

Cf. A000217, A000404, A006339, A016032, A025426, A273238.

Extensions

a(11)-a(16) from Giovanni Resta, Jul 04 2016