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 A242796 Least number k such that (k^k-n)/(k-n) is prime, or 0 if no such k exists. 0
 2, 4, 6, 5, 0, 13, 9, 11, 0, 12, 22, 13, 37, 28, 36, 0, 0, 171, 73, 85, 0, 0, 0, 29, 0, 0, 0, 0, 517, 35, 40, 49, 44, 49, 0, 41, 46, 40, 0, 0, 51, 0, 52, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2841, 0, 0, 0, 0, 0, 0, 67, 0, 64, 0, 199, 125, 221, 0, 0, 153, 113, 239, 0, 97, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(n)=0 is confirmed for k <= 5000. LINKS EXAMPLE (1^1-2)/(1-2) = 1 is not prime. (3^3-2)/(3-2) = 25 is not prime. (4^4-2)/(4-2) = 254/2 = 127 is prime. Thus a(2) = 4. PROG (PARI) a(n)=for(k=1, 5000, if(k!=n, s=(k^k-n)/(k-n); if(floor(s)==s, if(ispseudoprime(s), return(k))))); n=1; while(n<100, print(a(n)); n+=1) CROSSREFS Cf. A242787, A242788. Sequence in context: A099784 A082747 A127275 * A298527 A071288 A063892 Adjacent sequences:  A242793 A242794 A242795 * A242797 A242798 A242799 KEYWORD nonn,more,hard AUTHOR Derek Orr, May 22 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

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Last modified April 9 07:40 EDT 2020. Contains 333344 sequences. (Running on oeis4.)