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A260821 Least positive integer k for which n*2^(2^k) + 1 is composite. 0
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

Table of n, a(n) for n=1..87.

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.

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: A231990 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 10 13:35 EDT 2020. Contains 335576 sequences. (Running on oeis4.)