A233865 Numbers n such that sigma(sigma(n))+1 is prime.

1, 2, 5, 6, 11, 14, 15, 19, 20, 23, 26, 29, 36, 37, 41, 61, 63, 67, 68, 72, 74, 76, 82, 85, 86, 88, 90, 100, 102, 103, 104, 105, 107, 110, 113, 116, 117, 118, 120, 128, 129, 131, 133, 139, 141, 142, 144, 145, 146, 149, 153, 155, 157, 159, 161, 172, 174, 179, 181

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

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

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

Cf. A023194 (numbers n: sigma(n)is prime).

Cf. A228567 (primes: sigma(sigma(n))-sigma(n) is prime).

Sequence in context: A349158 A319242 A323398 * A090552 A024520 A340001

Adjacent sequences: A233862 A233863 A233864 * A233866 A233867 A233868

Keyword

nonn

Author

K. D. Bajpai, Dec 17 2013

Status

approved