OFFSET
1,1
COMMENTS
Corresponding primes are 3, 5, 73, 17, 6439469, 1772291, 411162217, 257, ...
a(n) = 4 if and only if 2^n + 1 is a Fermat prime (A019434).
LINKS
Iain Fox, Table of n, a(n) for n = 1..1000
EXAMPLE
a(5) = 115 because 1^5 + 5^5 + 23^5 = 6439469 is prime and 115 is the smallest number with this property.
MATHEMATICA
Table[SelectFirst[Range[10^4], PrimeQ[DivisorSigma[n, #] - #^n] &], {n, 69}] (* Michael De Vlieger, Aug 14 2017 *)
PROG
(PARI) a(n) = {my(k=1); while(!isprime(sigma(k, n)-k^n), k++); k; }
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Aug 12 2017
STATUS
approved