A258868 a(n) is the smallest integer >= a(n-1) such that prime(n)*2^a(n)-1 is a prime number.

1, 1, 2, 5, 26, 287, 356, 395, 544, 11008, 21957, 32125, 42450, 50867, 55408, 206970, 358276, 384287, 403461, 735802, 783831, 969795, 1192950, 1383108

Offset

1,3

Links

Table of n, a(n) for n=1..24.

K. Bonath, Riesel and Proth Prime Database (2015)

Example

2*2^1-1=3 prime so a(1)=1.
3*2^1-1=5 prime so a(2)=1.
5*2^1-1=9 composite, 5*2^2-1=19 prime so a(3)=2.

Maple

A258868 := proc(n)
    option remember;
    if n = 0 then
        0;
    else
        for a from procname(n-1) do
            ithprime(n)*2^a-1 ;
            if isprime(%) then
                return a;
            fi ;
        end do:
    end if;
end proc: # R. J. Mathar, Sep 23 2016

Mathematica

lst={1}; Do[x=Last[lst]; Label[begin];
If[PrimeQ[Prime[n]*2^x-1], AppendTo[lst, x], x=x+1; Goto[begin]], {n, 2, 9}]; lst
(* Ivan N. Ianakiev, Jun 19 2015 *)

Prog

(PARI) first(n)=my(t, p); vector(n, i, p=prime(i); while(!ispseudoprime(p<<t-1), t++); t) \\ Charles R Greathouse IV, Jul 03 2015

Crossrefs

Cf. A128979.

Sequence in context: A323299 A111195 A323293 * A322705 A167007 A064006

Adjacent sequences: A258865 A258866 A258867 * A258869 A258870 A258871

Keyword

nonn

Author

Pierre CAMI, Jun 13 2015

Status

approved