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 A090649 Numbers k such that 9^k + 2 is prime. 12
 0, 1, 2, 4, 5, 7, 12, 13, 18, 49, 55, 63, 193, 247, 610, 929, 2173, 3479, 5494, 11158, 16754, 30920, 47752, 50702, 53725, 68122, 89214, 180804 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS All terms are the exact halves of the even terms in A051783. - Alexander Adamchuk, Mar 02 2008 a(29) > 2*10^5. - Robert Price, Aug 18 2014 LINKS Henri Lifchitz and Renaud Lifchitz, PRP Records. EXAMPLE 9^13 + 2 = 2541865828331 is prime, so 13 is a term. MATHEMATICA Do[ If[ PrimeQ[9^n + 2], Print[n]], {n, 1, 2250}] (* Robert G. Wilson v, Feb 06 2004 *) PROG (PARI) for(i=0, 700, if(isprime(9^i+2), print(i))) CROSSREFS Cf. A051783 (3^k + 2 is prime), A087885 (5^k + 2 is prime). Sequence in context: A250006 A257033 A039674 * A154686 A165196 A007062 Adjacent sequences:  A090646 A090647 A090648 * A090650 A090651 A090652 KEYWORD nonn,more,hard AUTHOR Herman H. Rosenfeld (herm3(AT)pacbell.net), Feb 02 2004 EXTENSIONS More terms from mohammed bouayoun (bouyao(AT)wanadoo.fr), Feb 04 2004 Further terms from Robert G. Wilson v and Ray Chandler, Feb 06 2004 a(20) - a(22) found by Henri Lifchitz, a(23) found by Wojciech Florek. - Jason Earls, Feb 25 2008 a(24)-a(27) from A051783 by Ray Chandler, Aug 06 2011 a(28) from Robert Price, Aug 18 2014 Removal of erroneous term (97715) by Robert Price, Aug 19 2014 STATUS approved

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Last modified September 16 23:53 EDT 2021. Contains 347477 sequences. (Running on oeis4.)