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A228567 Primes expressible as sigma(sigma(n)) - sigma(n), in order of their occurrence. 4
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 (list; graph; refs; listen; history; text; internal format)
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: A191081 A079779 A083991 * A123988 A006628 A068682

Adjacent sequences:  A228564 A228565 A228566 * A228568 A228569 A228570

KEYWORD

nonn

AUTHOR

K. D. Bajpai, Nov 10 2013

STATUS

approved

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Last modified August 10 04:38 EDT 2020. Contains 336368 sequences. (Running on oeis4.)