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A128461 Numbers n such that 21^n - 2 is a prime. 12
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

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

LINKS

Table of n, a(n) for n=1..25.

MATHEMATICA

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

CROSSREFS

Cf. A084714 = smallest prime of the form (2n-1)^k - 2, or 0 if no such number exists. Cf. A128472 = smallest prime of the form (2n-1)^k - 2 for k>(2n-1), or 0 if no such number exists. Cf. A014224, A109080, A090669, A128455, A128457, A128458, A128459, A128460.

Sequence in context: A129211 A175204 A215533 * A011956 A018003 A328692

Adjacent sequences:  A128458 A128459 A128460 * A128462 A128463 A128464

KEYWORD

hard,more,nonn

AUTHOR

Alexander Adamchuk, Mar 14 2007

EXTENSIONS

2 more terms from Stefan Steinerberger, May 05 2007

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

STATUS

approved

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Last modified February 23 12:38 EST 2020. Contains 332159 sequences. (Running on oeis4.)