

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
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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

Table of n, a(n) for n=1..28.
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



