OFFSET
1,1
LINKS
K. D. Bajpai, Table of n, a(n) for n = 1..10000
EXAMPLE
a(4) = 24: sigma(sigma(n))-sigma(n)-1 = 168-60-1 = 107, which is prime.
a(10) = 45: sigma(sigma(n))-sigma(n)-1 = 168-78-1 = 89, which is prime.
MAPLE
with(numtheory): KD := proc() local a; a:= sigma(sigma(n))-sigma(n)-1; if isprime(a) then RETURN (n); fi; end: seq(KD(), n=1..500);
MATHEMATICA
Select[Range[300], PrimeQ[DivisorSigma[1, DivisorSigma[1, #]]-DivisorSigma[ 1, #]-1]&] (* Harvey P. Dale, Jun 04 2021 *)
CROSSREFS
KEYWORD
nonn,changed
AUTHOR
K. D. Bajpai, Nov 11 2013
STATUS
approved