A242017 Smallest prime factor of composites in the sequence A000051(n) = 2^n+1.

3, 3, 5, 3, 3, 5, 3, 17, 3, 5, 3, 3, 5, 3, 17, 3, 5, 3, 97, 3, 5, 3, 17, 3, 5, 3, 641, 3, 5, 3, 17, 3, 5, 3, 257, 3, 5, 3, 17, 3, 5, 3, 193, 3, 5, 3, 17, 3, 5, 3, 257, 3, 5, 3, 17, 3, 5, 3, 274177, 3, 5, 3

Offset: 1

Author

Felix Fröhlich, Aug 11 2014

Keywords

• nonn

Links

• Chai Wah Wu, Table of n, a(n) for n = 1..1786

Crossrefs

Cf. A000051, A023394, A242016.

Programs

Mathematica

FactorInteger[#][[1,1]]&/@Select[(2^Range[70]+1),CompositeQ] (* Harvey P. Dale, Feb 17 2017 *)

PARI

for(n=1, 1e2, if(!ispseudoprime(2^n+1), p=factor(2^n+1)[1, 1]; print1(p, ", ")))