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 A153442 Numbers k such that k^81*(k^81+1)+1 is prime. 6
 1, 209, 210, 842, 1176, 1358, 1370, 1608, 1707, 1845, 1850, 2594, 2880, 2882, 3123, 3384, 4085, 4457, 4469, 4808, 5090, 5186, 5516, 5529, 5867, 5991, 6123, 6144, 6606, 6906, 7001, 7019, 7119, 7430, 7541, 7719, 8031, 8463, 8471, 8486, 8595, 8609, 8627 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS It seems numbers of the form k^n*(k^n+1)+1 with n > 0, k > 1 may be primes only if n has the form 3^j. When n is even, k^(4*n)+k^(2*n)+1=(k^(2*n)+1)^2-(k^n)^2=(k^(2*n)+k^n+1)*(k^(2*n)-k^n+1) so composite. But why if n odd > 3 and not a power of 3, k^n*(k^n+1)+1 is always composite? LINKS Pierre CAMI, Table of n, a(n) for n=1,...,9439 MATHEMATICA k81Q[k_]:=Module[{k81=k^81}, PrimeQ[k81(k81+1)+1]]; Select[Range[9000], k81Q] (* Harvey P. Dale, Aug 28 2011 *) Select[Range[9000], PrimeQ[(#^81 (#^81 + 1)) + 1] &] (* Vincenzo Librandi, Jan 17 2015 *) CROSSREFS Sequence in context: A003740 A303688 A080532 * A159274 A330206 A025334 Adjacent sequences:  A153439 A153440 A153441 * A153443 A153444 A153445 KEYWORD nonn AUTHOR Pierre CAMI, Dec 26 2008 STATUS approved

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Last modified December 8 12:07 EST 2019. Contains 329862 sequences. (Running on oeis4.)