A134263 Numbers n such that sigma(n) and sigma(sigma(n)) are both perfect squares.

1, 343, 10680, 11448, 11832, 12760, 12920, 13470, 14360, 14526, 15064, 15198, 15486, 15752, 16194, 16264, 16376, 16390, 16472, 16910, 17110, 18734, 20366, 20654, 20822, 21094, 21122, 21158, 21598, 21843, 22899, 23403, 23691, 24695

Offset

1,2

Links

Giovanni Resta, Table of n, a(n) for n = 1..2000 (first 77 terms from Amiram Eldar)

Example

sigma(343) = 400 = 20^2 and sigma(400) = 961 = 31^2, so 343 belongs to the sequence.

Mathematica

l = {}; For[i = 1, i <= 10^6, i++, a = DivisorSigma[1, i]; b = DivisorSigma[1, a]; If[IntegerQ[a^(1/2)] && IntegerQ[b^(1/2)], l = Append[l, i]]]; l

Prog

(PARI) isok(n) = issquare(sigma(n)) && issquare(sigma(sigma(n))); \\ Michel Marcus, Aug 15 2019

Crossrefs

Subsequence of A006532.

Sequence in context: A017607 A277636 A224427 * A270820 A140250 A117197

Adjacent sequences: A134260 A134261 A134262 * A134264 A134265 A134266

Keyword

nonn

Author

Joseph L. Pe, Jan 14 2008

Status

approved