A234638 Numbers n for which sigma(sigma(n)) is odd.

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

Robert Israel, Table of n, a(n) for n = 1..10000

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

Cf. A000203, A028982, A066436, A234641.

Sequence in context: A163714 A333910 A307191 * A345888 A128223 A217258

Adjacent sequences: A234635 A234636 A234637 * A234639 A234640 A234641

Keyword

nonn

Author

M. F. Hasler, Dec 28 2013

Status

approved