OFFSET
1,2
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..5800
EXAMPLE
6 is in the sequence because sigma(sigma(6))+1= 29, which is prime.
19 is in the sequence because sigma(sigma(19))+1= 43, which is prime.
MAPLE
with(numtheory): KD := proc() local a; a:= sigma(sigma(n))+1; if isprime(a) then RETURN (n); fi; end: seq(KD(), n=1..500);
MATHEMATICA
Select[Range[200], PrimeQ[DivisorSigma[1, DivisorSigma[1, #]]+1]&] (* Harvey P. Dale, Jan 29 2021 *)
PROG
(PARI) is(n)=isprime(sigma(sigma(n))+1) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
K. D. Bajpai, Dec 17 2013
STATUS
approved