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A234639
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Numbers n for which sigma(sigma(sigma(n))) is odd.
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2
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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
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OFFSET
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1,2
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COMMENTS
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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.
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LINKS
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MAPLE
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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;
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MATHEMATICA
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sssoQ[n_]:=OddQ[Nest[DivisorSigma[1, #]&, n, 3]]; Select[Range[800], sssoQ] (* Harvey P. Dale, Jul 06 2019 *)
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PROG
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(PARI) for(n=1, 999, sigma(sigma(sigma(n)))%2 && print1(n", "))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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