A125049 a(1) = 1. If a(n) is prime, a(n+1) = 2*a(n); otherwise, a(n+1) = 2*a(n) + 1.

1, 3, 6, 13, 26, 53, 106, 213, 427, 855, 1711, 3423, 6847, 13695, 27391, 54783, 109567, 219134, 438269, 876539, 1753079, 3506159, 7012318, 14024637, 28049275, 56098551, 112197103, 224394207, 448788415, 897576831, 1795153663, 3590307326

Offset: 1

Author

Franklin T. Adams-Watters, Nov 17 2006

Keywords

• nonn

Crossrefs

Cf. A125050.

Programs

Maple

a := proc(n)
   local c, k:
   c, k := 1, 1:
   while c < n do
      if isprime(k) then k := 2*k: else k := 2*k+1: fi:
      c := c+1: od:
   k: end: # Lorenzo Sauras Altuzarra, Jan 02 2023

Mathematica

NestList[If[PrimeQ[#],2#,2#+1]&,1,40] (* Harvey P. Dale, Sep 01 2017 *)

Formula

a(n) = floor(c*2^n), where c = 0.8359335658... - Lorenzo Sauras Altuzarra, Jan 01 2023