A231587 Numbers k such that sigma(sigma(k)) - sigma(k) + 1 is prime.

2, 4, 5, 6, 9, 11, 12, 13, 14, 15, 16, 19, 23, 24, 25, 29, 34, 37, 38, 42, 43, 48, 53, 54, 56, 57, 59, 60, 62, 63, 64, 67, 69, 73, 74, 75, 76, 77, 78, 79, 80, 81, 84, 85, 87, 88, 90, 91, 92, 95, 103, 104, 105, 107, 109, 111, 112, 113, 116, 118, 120, 122, 123, 124

Offset

1,1

Links

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

Example

a(3) = 5: sigma(sigma(5))-sigma(5)+1 = 12-6+1 = 7, which is prime.
a(10) = 15: sigma(sigma(15))-sigma(15)+1 = 60-24+1 = 37, 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);

Crossrefs

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

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

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

Sequence in context: A285163 A015834 A252041 * A050011 A030303 A047315

Adjacent sequences: A231584 A231585 A231586 * A231588 A231589 A231590

Keyword

nonn

Author

K. D. Bajpai, Nov 11 2013

Status

approved