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
Antti Karttunen, Table of n, a(n) for n = 1..3718 (first 1000 terms from Harry J. Smith)
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
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Jan 26 2002
EXTENSIONS
More terms from Lior Manor, Feb 06 2002
STATUS
approved