1,7

Sierpinski showed that a(n) = -1 infinitely often. John Selfridge showed that a(78557) = -1 and it is conjectured that a(n) >= 0 for all n < 78557.

T. D. Noe, Table of n, a(n) for n=1..1000

Ray Ballinger and Wilfrid Keller, The Sierpinski Problem: Definition and Status

Seventeen or Bust, A Distributed Attack on the Sierpinski problem

1*(2^0)+1=2 is prime, so a(1)=0;

3*(2^1)+1=5 is prime, so a(3)=1;

For n=7, 7+1 and 7*2+1 are composite, but 7*2^2+1=29 is prime, so a(7)=2.

Do[m = 0; While[ !PrimeQ[n*2^m + 1], m++ ]; Print[m], {n, 1, 110} ]

For the corresponding primes see A050921. Cf. A103964, A040081.

Sequence in context: A130538 A078659 A079690 this_sequence A019269 A035155 A090584

Adjacent sequences: A040073 A040074 A040075 this_sequence A040077 A040078 A040079

nonn,easy,nice

David W. Wilson (davidwwilson(AT)comcast.net)