0,2

Inverse of the Jacobsthal sequence (A001045), nearly, since a(A001045(n))=n except for n=1 (see A130250 for another version). a(n)+1 is equal to the partial sum of the Jacobsthal indicator sequence (see A105348).

Table of n, a(n) for n=0..100.

a(n)=floor(log_2(3n+1)). Also true: a(n)=A130250(n+1)-1=A130253(n)-1. G.f.: g(x)=1/(1-x)*sum{k>=1, x^A001045(k)}.

a(12)=5, since A001045(5)=11<=12, but A001045(6)=21>12.

For partial sums see A130251. Other related sequences A130250, A130253, A105348. A001045, A130233, A130241.

Sequence in context: A143489 A274102 A261100 * A061071 A122258 A263089

Adjacent sequences: A130246 A130247 A130248 * A130250 A130251 A130252

nonn

Hieronymus Fischer, May 20 2007

approved