a(n) = [x^n] Product_{k=1..n} (1 + x^k)^(n-k+1).

Terms

a(0) =1a(1) =1a(2) =2a(3) =8a(4) =31a(5) =124a(6) =515a(7) =2166a(8) =9182a(9) =39195a(10) =168216a(11) =725043a(12) =3136223a(13) =13606891a(14) =59187790a(15) =258034685a(16) =1127137141a(17) =4932071321a(18) =21614913239a(19) =94859273448a(20) =416820578198