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A134263 Numbers n such that sigma(n) and sigma(sigma(n)) are both perfect squares. 1
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

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

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Last modified August 13 05:18 EDT 2022. Contains 356077 sequences. (Running on oeis4.)