A050921 Smallest prime of form n*2^m+1, m >= 0, or 0 if no such prime exists.

2, 3, 7, 5, 11, 7, 29, 17, 19, 11, 23, 13, 53, 29, 31, 17, 137, 19, 1217, 41, 43, 23, 47, 97, 101, 53, 109, 29, 59, 31, 7937, 257, 67, 137, 71, 37, 149, 1217, 79, 41, 83, 43, 173, 89, 181, 47

Offset: 1

Author

N. J. A. Sloane, Dec 30 1999

Keywords

• nonn
• easy
• nice

Comments

Primes arising from A040076 (or 0 if no such prime exists).

Or: Starting with x=n+1, the first prime created by iterating the map x-> 2*x-1. - Kevin L. Schwartz and Christian N. K. Anderson, May 13 2013

Links

• R. J. Mathar, Table of n, a(n) for n = 1..382

Crossrefs

Cf. A040076, A034694, A038699.

Programs

Maple

A050921 := proc(n)
    for m from 0 do
        if isprime(n*2^m+1) then
            return n*2^m+1 ;
        end if;
    end do;
end proc; # R. J. Mathar, Jun 01 2013

Mathematica

Do[m = 0; While[ !PrimeQ[n*2^m + 1], m++ ]; Print[n*2^m + 1], {n, 1, 47} ]

Extensions

The next term (47*2^583 + 1) is too large to show.