Partial sums of A000238.

Terms

a(0) =1a(1) =2a(2) =5a(3) =13a(4) =40a(5) =131a(6) =481a(7) =1857a(8) =7600a(9) =32235a(10) =141203a(11) =633383a(12) =2899885a(13) =13498337a(14) =63734268a(15) =304606922a(16) =1471339736a(17) =7173341171a(18) =35262128485a(19) =174617051093a(20) =870425605393