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A228434 Primes expressible as sigma(n) + sigma(sigma(n)), in order of their occurrence. 1
2, 7, 11, 23, 127, 167, 137, 269, 547, 547, 383, 547, 269, 431, 547, 547, 293, 383, 431, 1171, 1039, 1171, 641, 1039, 1103, 1171, 887, 1361, 2551, 1861, 3001, 2753, 1193, 2963, 1499, 2153, 2753, 2551, 2963, 4327, 5281, 1823, 2963, 4219, 4327, 3593, 3583, 6763 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..2450

EXAMPLE

a(6)= 167: sigma(32)+sigma(sigma(32))= 63+104= 167, which is prime.

a(11)= 383: sigma(93)+sigma(sigma(93))= 128+255= 383, which is prime.

MAPLE

with(numtheory):KD := proc() local a; a:= sigma(n)+sigma(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))).

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

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

Sequence in context: A217304 A179876 A088179 * A031873 A075356 A235355

Adjacent sequences:  A228431 A228432 A228433 * A228435 A228436 A228437

KEYWORD

nonn

AUTHOR

K. D. Bajpai, Nov 10 2013

STATUS

approved

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Last modified January 20 17:44 EST 2018. Contains 297961 sequences.