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A066961 Numbers n such that sigma(n) divides sigma(sigma(n)). 5
1, 5, 12, 54, 56, 87, 95, 276, 308, 427, 429, 446, 455, 501, 581, 611, 9120, 9180, 9504, 9720, 9960, 10296, 10620, 10740, 10824, 11070, 11310, 11480, 11484, 11556, 11628, 11748, 11934, 11960, 12024, 12036, 12072, 12084, 12376, 12460, 12510, 12570 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Is this sequence finite?

These are numbers k such that sigma(k) is a multiply-perfect number (A007691). - Ivan N. Ianakiev, Sep 13 2016

LINKS

Harry J. Smith, Table of n, a(n) for n = 1..1000

EXAMPLE

12 is in the sequence since sigma(12) = 1 + 2 + 3 + 4 + 6 + 12 = 28 divides sigma(28) = 1 + 2 + 4 + 7 + 14 + 28 = 56. - Michael B. Porter, Sep 22 2016

MAPLE

with(numtheory): A066961:=n->`if`(sigma(sigma(n)) mod sigma(n) = 0, n, NULL): seq(A066961(n), n=1..2*10^4); # Wesley Ivan Hurt, Sep 22 2016

MATHEMATICA

Select[Range[30000], Divisible[DivisorSigma[1, DivisorSigma[1, #]], DivisorSigma[1, #]] &] (* Ivan N. Ianakiev, Sep 13 2016 *)

PROG

(PARI) { n=0; for (m=1, 10^10, if (sigma(sigma(m)) % sigma(m) == 0, write("b066961.txt", n++, " ", m); if (n==1000, return)) ) } \\ Harry J. Smith, Apr 11 2010

(PARI) isok(n) = (s=sigma(n)) && ((sigma(s) % s) == 0); \\ Michel Marcus, Sep 17 2016

(MAGMA) [n: n in [1..13000] | (SumOfDivisors(SumOfDivisors(n)) mod SumOfDivisors(n) eq 0)]; // Vincenzo Librandi, Sep 13 2016

CROSSREFS

Cf. A000203, A007691.

Sequence in context: A188118 A062978 A189421 * A179994 A131549 A111904

Adjacent sequences:  A066958 A066959 A066960 * A066962 A066963 A066964

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Jan 26 2002

EXTENSIONS

More terms from Lior Manor, Feb 06 2002

STATUS

approved

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Last modified October 19 04:40 EDT 2019. Contains 328211 sequences. (Running on oeis4.)