

A262994


Smallest number k>2 such that k*2^n1 is a prime number.


2



3, 3, 3, 3, 4, 3, 3, 5, 7, 5, 3, 5, 9, 5, 4, 8, 4, 3, 28, 14, 7, 26, 13, 39, 22, 11, 16, 8, 4, 20, 10, 5, 6, 3, 24, 12, 6, 3, 25, 24, 12, 6, 3, 14, 7, 20, 10, 5, 19, 11, 21, 20, 10, 5, 3, 32, 16, 8, 4, 17, 24, 12, 6, 3, 67, 63, 43, 63, 40, 20, 10, 5, 15, 12, 6, 3
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OFFSET

1,1


COMMENTS

If k=2^j then n+j is a Mersenne exponent.
a(n)=3 if and only if 3*2^n1 is a prime; that is, n belongs to A002235.  Altug Alkan, Oct 08 2015


LINKS

Pierre CAMI, Table of n, a(n) for n = 1..10000


EXAMPLE

3*2^11=5 prime so a(1)=3;
3*2^21=11 prime so a(2)=3;
3*2^31=23 prime so a(3)=3.


MATHEMATICA

a[n_] := For[k = 3, True, k++, If[PrimeQ[k*2^n  1], Return[k]]]; Table[a[n], {n, 1, 100}] (* JeanFrançois Alcover, Oct 07 2015 *)


PROG

(PARI) a(n) = {k=3; while (! isprime(k*2^n1), k++); k; } \\ Michel Marcus, Oct 08 2015


CROSSREFS

Cf. A247202, A002235.
Sequence in context: A276863 A227321 A309555 * A179847 A035936 A006671
Adjacent sequences: A262991 A262992 A262993 * A262995 A262996 A262997


KEYWORD

nonn


AUTHOR

Pierre CAMI, Oct 07 2015


STATUS

approved



