A113930 Numbers k such that sigma(k) and phi(k) are both triangular numbers.

1, 2, 22, 3051, 3219, 3393, 5057, 8653, 75618, 95675, 100503, 102949, 104714, 287826, 438547, 522339, 537159, 688050, 2191200, 2317118, 2418548, 2507683, 2599128, 3212964, 4534573, 5367797, 6047913, 6302639, 7689149, 13758296, 14380145, 15342050, 16148979

Offset: 1

Author

Giovanni Resta, Jan 30 2006

Keywords

• nonn

Comments

phi(k) = A000010(k) is the Euler totient function, while sigma(k) = A000203(k) is the sum of divisors of k.

Examples

			sigma(100503) = 156520 = T(559) and phi(100503) = 61776 = T(351).
		

Links

• Donovan Johnson, Table of n, a(n) for n = 1..207 (terms < 10^12)

Crossrefs

Cf. A000010, A000203, A000217.

Subsequence of A045746.

Programs

PARI

isok(n) = ispolygonal(sigma(n), 3) && ispolygonal(eulerphi(n), 3); \\ Michel Marcus, Jan 26 2014