A231627 Numbers n such that sigma(sigma(n)) - sigma(n) - 1 is prime.

3, 5, 20, 24, 26, 29, 38, 41, 44, 45, 54, 56, 59, 60, 65, 78, 80, 81, 83, 87, 90, 92, 95, 101, 102, 108, 110, 114, 120, 122, 123, 135, 136, 137, 142, 143, 146, 147, 150, 153, 158, 159, 164, 167, 168, 174, 176, 177, 178, 180, 184, 185, 187, 190, 194, 197, 203, 209

Offset

1,1

Links

K. D. Bajpai, Table of n, a(n) for n = 1..10000

Example

a(4)= 24: sigma(sigma(n))-sigma(n)-1= 168-60-1= 107, which is prime.
a(10)= 45: sigma(sigma(n))-sigma(n)-1= 168-78-1= 89, which is prime.

Maple

with(numtheory): KD := proc() local a; a:= sigma(sigma(n))-sigma(n)-1; if isprime(a) then RETURN (n); fi; end: seq(KD(), n=1..500);

Mathematica

Select[Range[300], PrimeQ[DivisorSigma[1, DivisorSigma[1, #]]-DivisorSigma[ 1, #]-1]&] (* Harvey P. Dale, Jun 04 2021 *)

Crossrefs

Cf. A000203 (a(n): sigma(n)).

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

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

Cf. A231587 (numbers n: sigma(sigma(n))-sigma(n)+1 is prime).

Sequence in context: A062577 A144743 A171862 * A261116 A295361 A295357

Adjacent sequences: A231624 A231625 A231626 * A231628 A231629 A231630

Keyword

nonn

Author

K. D. Bajpai, Nov 11 2013

Status

approved