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

Author

M. F. Hasler, Dec 28 2013

Keywords

• nonn

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

Examples

			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.
		

Links

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

Crossrefs

Cf. A000203, A028982, A066436, A234641.

Programs

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 *)

PARI

is(n)=bittest(sigma(sigma(n)),0)

PARI

for(n=1,999,is(n)&&print1(n","))