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A242922
Least number k such that (k^n+n^k)/(k+n) is prime or 0 if no such k exists.
1
0, 2, 8, 0, 0, 0, 43, 3, 0, 0, 0, 0, 21, 0, 0, 0, 0, 0, 0, 0, 13, 0, 47, 0, 287, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0, 23, 0, 0, 0, 0, 0, 77, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1795, 0, 0, 0, 0, 0, 0, 0, 0, 0, 53, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
OFFSET
1,2
COMMENTS
If a(i) = j, then a(j) = i for any i and j.
a(n) = 0 is definite for n=1. For other n-values where a(n) = 0, it is confirmed for k <= 5000. These are only conjectural.
EXAMPLE
(8^3+3^8)/(8+3) = 7073/11 = 643 is prime. Thus, a(3) = 8 and a(8) = 3.
PROG
(PARI) a(n)=for(k=1, 5000, s=(k^n+n^k)/(k+n); if(floor(s)==s, if(ispseudoprime(s), return(k))))
n=1; while(n<100, print(a(n)); n+=1)
CROSSREFS
Sequence in context: A246516 A095217 A230915 * A242530 A073410 A021361
KEYWORD
nonn,hard,more
AUTHOR
Derek Orr, May 26 2014
EXTENSIONS
We don't normally allow conjectural terms, except in special circumstances. This is one of those exceptions, for if we included only terms that are known for certain, not much of this sequence would remain. - N. J. A. Sloane, May 31 2014
STATUS
approved