G.f.: Sum_{n>=0} (1 + x^n)^n * x^n / (1-x)^(n+1).

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =21a(5) =42a(6) =85a(7) =169a(8) =333a(9) =655a(10) =1286a(11) =2519a(12) =4935a(13) =9675a(14) =18982a(15) =37285a(16) =73346a(17) =144509a(18) =285158a(19) =563546a(20) =1115309a(21) =2210243a(22) =4385443a(23) =8710876a(24) =17319387a(25) =34464792a(26) =68634821a(27) =136771603a(28) =272703704a(29) =543995341