Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #9 Jan 29 2021 15:17:02
%S 1,2,5,6,11,14,15,19,20,23,26,29,36,37,41,61,63,67,68,72,74,76,82,85,
%T 86,88,90,100,102,103,104,105,107,110,113,116,117,118,120,128,129,131,
%U 133,139,141,142,144,145,146,149,153,155,157,159,161,172,174,179,181
%N Numbers n such that sigma(sigma(n))+1 is prime.
%H K. D. Bajpai, <a href="/A233865/b233865.txt">Table of n, a(n) for n = 1..5800</a>
%e 6 is in the sequence because sigma(sigma(6))+1= 29, which is prime.
%e 19 is in the sequence because sigma(sigma(19))+1= 43, which is prime.
%p with(numtheory): KD := proc() local a; a:= sigma(sigma(n))+1; if isprime(a) then RETURN (n); fi; end: seq(KD(),n=1..500);
%t Select[Range[200],PrimeQ[DivisorSigma[1,DivisorSigma[1,#]]+1]&] (* _Harvey P. Dale_, Jan 29 2021 *)
%o (PARI) is(n)=isprime(sigma(sigma(n))+1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A000203 (sigma(n): sum of divisors of n).
%Y Cf. A019279 (superperfect numbers: sigma(sigma(n))).
%Y Cf. A023194 (numbers n: sigma(n)is prime).
%Y Cf. A228567 (primes: sigma(sigma(n))-sigma(n) is prime).
%K nonn
%O 1,2
%A _K. D. Bajpai_, Dec 17 2013