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A138049
Numbers k such that 21^k + 2 is prime.
7
0, 1, 2, 4, 7, 24, 40, 112, 310, 1026, 1286, 36566, 43717, 53753
OFFSET
1,3
COMMENTS
No further terms < 100000. - Ray Chandler, Aug 11 2011
a(15) > 2*10^5. - Robert Price, Jul 14 2015
LINKS
Henri & Renaud Lifchitz, PRP Records.
MATHEMATICA
Do[ f = 21^n + 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n, 1, 310} ]
PROG
(PARI) is(n)=ispseudoprime(21^n+2) \\ Charles R Greathouse IV, Feb 17 2017
CROSSREFS
Cf. A051783 (k such that 3^k + 2 is prime).
Cf. A087885 (k such that 5^k + 2 is prime).
Sequence in context: A071795 A059501 A353986 * A099387 A359095 A036440
KEYWORD
nonn,more
AUTHOR
Alexander Adamchuk, Mar 02 2008
EXTENSIONS
1026 from Herman Jamke (hermanjamke(AT)fastmail.fm), May 01 2008
1286 and 36566 from Ray Chandler, Jul 31 2011
43717 from Ray Chandler, Aug 01 2011
53753 from Ray Chandler, Aug 02 2011
STATUS
approved