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 A123599 Smallest generalized Fermat prime of the form a^(2^n) + 1, where base a>1 is an integer; or -1 if no such prime exists. 6
 3, 5, 17, 257, 65537, 185302018885184100000000000000000000000000000001 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS First 5 terms {3, 5, 17, 257, 65537} = A019434 are the Fermat primes of the form 2^(2^n) + 1. Note that for all currently known a(n) up to n = 17 last digit is 7 or 1 (except a(0) = 3 and a(1) = 5). Corresponding least bases a>1 such that a^(2^n) + 1 is prime are listed in A056993. The last-digit behavior clearly continues since, for any a, we have that a^(2^2) will be either 0 or 1 modulo 5. So for n >= 2, a(n) is 1 or 2 modulo 5, and odd. - Jeppe Stig Nielsen, Nov 16 2020 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 0..9 Eric Weisstein's World of Mathematics, Generalized Fermat Number. MATHEMATICA Do[f=Min[Select[ Table[ i^(2^n) + 1, {i, 2, 500} ], PrimeQ]]; Print[{n, f}], {n, 0, 9}] CROSSREFS Cf. A000215, A019434, A056993. Cf. A006093, A005574, A000068, A006314, A006313, A006315, A006316, A056994, A056995, A057465, A057002. Sequence in context: A000215 A339344 A263539 * A100270 A016045 A349121 Adjacent sequences:  A123596 A123597 A123598 * A123600 A123601 A123602 KEYWORD nonn AUTHOR Alexander Adamchuk, Nov 14 2006 STATUS approved

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Last modified November 30 16:47 EST 2021. Contains 349423 sequences. (Running on oeis4.)