A260821 - OEIS

A260821

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

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

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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 *)