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A134263 Numbers n such that sigma(n) and sigma(sigma(n)) are both perfect squares. 0
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..34.

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

CROSSREFS

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

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Last modified October 17 22:48 EDT 2018. Contains 316297 sequences. (Running on oeis4.)