OFFSET
2,1
COMMENTS
Conjecture: sequence is defined for any n>=2.
FORMULA
a(n) = A085398(2*prime(n)) for n >= 2. - Jinyuan Wang, Dec 17 2022
EXAMPLE
(2^prime(2)+1)/(2+1) = 3 is prime, so a(2)=2;
(2^prime(10)+1)/(2+1) = 178956971 has a factor of 59;
(3^prime(10)+1)/(3+1) = 17157594341221 has a factor of 523;
...
(7^prime(10)+1)/(7+1) = 402488219476647465854701 is prime, so a(10)=7.
MATHEMATICA
Do[p=Prime[k]; n=2; cp=(n^p+1)/(n+1); While[ !PrimeQ[cp], n=n+1; cp=(n^p+1)/(n+1)]; Print[n], {k, 2, 200}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Lei Zhou, Feb 23 2005
STATUS
approved