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 A260821 Least positive integer k for which n*2^(2^k) + 1 is composite. 1
 5, 1, 2, 2, 1, 1, 3, 1, 2, 2, 1, 1, 2, 1, 3, 1, 1, 2, 1, 1, 1, 3, 1, 2, 3, 1, 3, 3, 1, 1, 1, 1, 1, 2, 1, 1, 5, 1, 2, 1, 1, 1, 2, 1, 2, 1, 1, 4, 2, 1, 1, 1, 1, 1, 1, 1, 2, 3, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 2, 2, 1, 1, 2, 1, 1, 1, 1, 3, 2, 1, 1, 1, 1, 2, 1, 1, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(n) = 1 for nonzero n in A045751. - Michel Marcus, Aug 01 2015 LINKS Paolo Xausa, Table of n, a(n) for n = 1..10000 EXAMPLE a(7)=3 because 7*2^2 + 1 = 29 is prime and 7*2^(2^2) + 1 = 113 is also prime, while 7*2^(2^3) + 1 = 11*163. MATHEMATICA A260821[n_] := Module[{k = 0}, While[PrimeQ[n*2^(2^++k) + 1]]; k]; Array[A260821, 100] (* Paolo Xausa, Jan 31 2024 *) PROG (PARI) a(n) = {k = 1; while (isprime(n*2^2^k+1), k++); k; } \\ Michel Marcus, Aug 01 2015 CROSSREFS Cf. A045751, A005098. Sequence in context: A369915 A010131 A176323 * A068115 A068139 A140987 Adjacent sequences: A260818 A260819 A260820 * A260822 A260823 A260824 KEYWORD nonn AUTHOR Marco Ripà, Jul 31 2015 EXTENSIONS More terms from Michel Marcus, Aug 01 2015 STATUS approved

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Last modified July 15 00:37 EDT 2024. Contains 374323 sequences. (Running on oeis4.)