A138049 Numbers k such that 21^k + 2 is prime.

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

Table of n, a(n) for n=1..14.

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).

Cf. A090649, A109076, A113480, A138048, A138050, A138051, A087886, A113481.

Sequence in context: A071795 A059501 A353986 * A099387 A359095 A036440

Adjacent sequences: A138046 A138047 A138048 * A138050 A138051 A138052

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