Dear SeqFans, not sure that this is worth submitting to OEIS... and I dare suggest it here as puzzle: 1,3,6,11,23,47,97,197,401,813,1647,3330,6725,13569,27355,55109,... Two hints: 1. a(n+1)/a(n) => 2. 2. It's related to binary expansion of fractions. PS I have exact formula and short Mma - ready to submit to OEIS if anyone of Eds will recommmend. Thx, Zak