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A128461
Numbers k such that 21^k - 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
OFFSET
1,2
COMMENTS
a(26) > 2*10^5. - Robert Price, Jul 14 2015
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).
Cf. A128472 (smallest prime of the form (2n-1)^k - 2 for k > (2n-1)).
Sequence in context: A129211 A175204 A215533 * A011956 A371358 A018003
KEYWORD
nonn,hard,more
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