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

KEYWORD

nonn

AUTHOR

M. F. Hasler, Dec 28 2013

STATUS

approved