a(n) = Sum_{d|n} binomial(d+n-1,n).

Terms

a(0) =1a(1) =4a(2) =11a(3) =41a(4) =127a(5) =498a(6) =1717a(7) =6610a(8) =24366a(9) =93391a(10) =352717a(11) =1358826a(12) =5200301a(13) =20097076a(14) =77562773a(15) =300786339a(16) =1166803111a(17) =4539163784a(18) =17672631901a(19) =68933291834a(20) =269129233484