A228434 Primes expressible as sigma(n) + sigma(sigma(n)), in order of their occurrence.

2, 7, 11, 23, 127, 167, 137, 269, 547, 547, 383, 547, 269, 431, 547, 547, 293, 383, 431, 1171, 1039, 1171, 641, 1039, 1103, 1171, 887, 1361, 2551, 1861, 3001, 2753, 1193, 2963, 1499, 2153, 2753, 2551, 2963, 4327, 5281, 1823, 2963, 4219, 4327, 3593, 3583, 6763

Offset: 1

Author

K. D. Bajpai, Nov 10 2013

Keywords

• nonn

Examples

			a(6)= 167: sigma(32)+sigma(sigma(32))= 63+104= 167, which is prime.
a(11)= 383: sigma(93)+sigma(sigma(93))= 128+255= 383, which is prime.
		

Links

• K. D. Bajpai, Table of n, a(n) for n = 1..2450

Crossrefs

Cf. A000203 (sigma(n): sum of divisors of n).

Cf. A019279 (superperfect numbers: sigma(sigma(n))).

Cf. A033632 (numbers n: sigma(n)is prime).

Cf. A051027 (a(n)= sigma(sigma(n))).

Programs

Maple

with(numtheory):KD := proc() local a; a:= sigma(n)+sigma(sigma(n));if isprime(a) then RETURN (a);fi;end:seq(KD(),n=1..5000);