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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
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
Sequence in context: A369915 A010131 A176323 * A068115 A068139 A140987
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.)