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

Author

Alexander Adamchuk, Mar 02 2008

Keywords

• nonn
• more

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.

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.

Programs

Mathematica

Do[ f = 21^n + 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n, 1, 310} ]

PARI

is(n)=ispseudoprime(21^n+2) \\ Charles R Greathouse IV, Feb 17 2017

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