A234639 Numbers n for which sigma(sigma(sigma(n))) is odd.

1, 2, 4, 16, 25, 29, 50, 64, 99, 100, 104, 116, 125, 137, 243, 279, 281, 309, 317, 343, 361, 373, 396, 408, 424, 440, 456, 461, 464, 486, 488, 500, 509, 519, 534, 538, 548, 558, 568, 585, 590, 603, 618, 632, 638, 641, 646, 686, 693, 703, 709, 718, 725, 746, 747, 783

Offset

1,2

Comments

Also, numbers such that sigma(sigma(n)) is in A028982, i.e., a square or twice a square.

See A234640 for the subsequence of odd terms. See also A234638, and A234641 with further cross references.

Links

R. J. Mathar, Table of n, a(n) for n = 1..404

Maple

n := 1 :
for k from 1 to 10000 do
    numtheory[sigma](k) ;
    numtheory[sigma](%) ;
    numtheory[sigma](%) ;
    if type(%, 'odd') then
        printf("%d %d\n", n, k) ;
        n := n+1 ;
    end if;
end do: # R. J. Mathar, Oct 21 2014

Mathematica

sssoQ[n_]:=OddQ[Nest[DivisorSigma[1, #]&, n, 3]]; Select[Range[800], sssoQ] (* Harvey P. Dale, Jul 06 2019 *)

Prog

(PARI) for(n=1, 999, sigma(sigma(sigma(n)))%2 && print1(n", "))

Crossrefs

Sequence in context: A239363 A045541 A372898 * A358000 A275674 A225983

Adjacent sequences: A234636 A234637 A234638 * A234640 A234641 A234642

Keyword

nonn

Author

M. F. Hasler, Dec 28 2013

Status

approved