A228567 - OEIS

A228567

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

3, 7, 17, 31, 31, 41, 23, 73, 127, 73, 89, 127, 463, 523, 241, 523, 157, 241, 523, 463, 211, 257, 131, 983, 379, 1153, 311, 1153, 83, 983, 521, 4339, 4339, 113, 8893, 4339, 4339, 1093, 4339, 769, 2851, 8893, 4339, 1429, 1097, 4339, 1093, 4339, 8893, 4339, 8893

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OFFSET

1,1

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..2200

EXAMPLE

a(9)= 127: sigma(sigma(93))-sigma(93)= 255-128= 127, which is prime.

a(11)= 89: sigma(sigma(98))-sigma(98)= 260-171= 89, which is prime.

MAPLE

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

CROSSREFS

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

Cf. A019279 (superperfect numbers: sigma(sigma(n))=2n).

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

Cf. A051027 (sigma(sigma(n))).

Sequence in context: A079779 A083991 A372082 * A123988 A006628 A068682

Adjacent sequences: A228564 A228565 A228566 * A228568 A228569 A228570

KEYWORD

nonn

AUTHOR

K. D. Bajpai, Nov 10 2013

STATUS

approved