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A233865
Numbers n such that sigma(sigma(n))+1 is prime.
1
1, 2, 5, 6, 11, 14, 15, 19, 20, 23, 26, 29, 36, 37, 41, 61, 63, 67, 68, 72, 74, 76, 82, 85, 86, 88, 90, 100, 102, 103, 104, 105, 107, 110, 113, 116, 117, 118, 120, 128, 129, 131, 133, 139, 141, 142, 144, 145, 146, 149, 153, 155, 157, 159, 161, 172, 174, 179, 181
OFFSET
1,2
LINKS
EXAMPLE
6 is in the sequence because sigma(sigma(6))+1= 29, which is prime.
19 is in the sequence because sigma(sigma(19))+1= 43, which is prime.
MAPLE
with(numtheory): KD := proc() local a; a:= sigma(sigma(n))+1; if isprime(a) then RETURN (n); fi; end: seq(KD(), n=1..500);
MATHEMATICA
Select[Range[200], PrimeQ[DivisorSigma[1, DivisorSigma[1, #]]+1]&] (* Harvey P. Dale, Jan 29 2021 *)
PROG
(PARI) is(n)=isprime(sigma(sigma(n))+1) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
Cf. A000203 (sigma(n): sum of divisors of n).
Cf. A019279 (superperfect numbers: sigma(sigma(n))).
Cf. A023194 (numbers n: sigma(n)is prime).
Cf. A228567 (primes: sigma(sigma(n))-sigma(n) is prime).
Sequence in context: A349158 A319242 A323398 * A090552 A024520 A340001
KEYWORD
nonn
AUTHOR
K. D. Bajpai, Dec 17 2013
STATUS
approved