A234640 -id:A234640 - cp's OEIS frontend

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

M. F. Hasler, Dec 28 2013

nonn

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.

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

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

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

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

cp's OEIS Frontend

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

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