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

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =75a(5) =274a(6) =1016a(7) =3807a(8) =14377a(9) =54627a(10) =208584a(11) =799669a(12) =3076167a(13) =11867511a(14) =45897145a(15) =177888715a(16) =690770763a(17) =2686879415a(18) =10466761637a(19) =40828165464a(20) =159453481037a(21) =623427464093