A128461 Numbers k such that 21^k - 2 is a prime.

1, 2, 4, 10, 21, 25, 27, 32, 60, 88, 106, 120, 146, 264, 828, 965, 1944, 4822, 12089, 14427, 17354, 42335, 46395, 58348, 190632

Offset: 1

Author

Alexander Adamchuk, Mar 14 2007

Keywords

• nonn
• hard
• more

Comments

a(26) > 2*10^5. - Robert Price, Jul 14 2015

Crossrefs

Cf. A084714 (smallest prime of the form (2n-1)^k - 2).

Cf. A128472 (smallest prime of the form (2n-1)^k - 2 for k > (2n-1)).

Cf. A014224, A109080, A090669, A128455, A128457, A128458, A128459, A128460.

Programs

Mathematica

Do[ f = 21^n - 2; If[ PrimeQ[ f ], Print[ {n, f} ] ], {n,1,1000} ]

Extensions

2 more terms from Stefan Steinerberger, May 05 2007

a(17)-a(25) from Robert Price, Jul 14 2015