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A228567
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Primes expressible as sigma(sigma(n)) - sigma(n), in order of their occurrence.
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4
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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
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1,1
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LINKS
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EXAMPLE
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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.
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MAPLE
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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);
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CROSSREFS
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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).
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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