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A234638
Numbers n for which sigma(sigma(n)) is odd.
3
1, 3, 7, 10, 17, 21, 22, 30, 31, 46, 51, 52, 55, 66, 70, 71, 81, 93, 94, 97, 106, 115, 119, 127, 138, 154, 156, 165, 170, 199, 210, 213, 214, 217, 232, 235, 241, 253, 265, 282, 291, 298, 310, 318, 322, 337, 343, 345, 357, 364, 374, 381, 382, 385
OFFSET
1,2
COMMENTS
See A234641 for numbers n such that n^2 is in this sequence.
The primes in this sequence are 3 and A066436. - Robert Israel, Apr 05 2019
LINKS
EXAMPLE
3 is in the sequence because sigma(sigma(3)) = sigma(4) = 7, which is odd.
7 is in the sequence because sigma(sigma(7)) = sigma(8) = 15, which is odd.
8 is not in the sequence because sigma(sigma(8)) = sigma(15) = 24, which is even.
MAPLE
filter:= proc(n) uses numtheory; sigma(sigma(n))::odd end proc:
select(filter, [$1..1000]); # Robert Israel, Apr 05 2019
MATHEMATICA
Select[Range[400], OddQ[DivisorSigma[1, DivisorSigma[1, #]]] &] (* Alonso del Arte, Dec 29 2013 *)
PROG
(PARI) is(n)=bittest(sigma(sigma(n)), 0)
(PARI) for(n=1, 999, is(n)&&print1(n", "))
CROSSREFS
KEYWORD
nonn
AUTHOR
M. F. Hasler, Dec 28 2013
STATUS
approved