a(n) = n * Sum_{d|n} binomial(d+n-2,n-1)/d.

Terms

a(0) =1a(1) =4a(2) =9a(3) =32a(4) =75a(5) =318a(6) =931a(7) =3712a(8) =13014a(9) =50110a(10) =184767a(11) =715656a(12) =2704169a(13) =10454976a(14) =40126395a(15) =155462016a(16) =601080407a(17) =2335849578a(18) =9075135319a(19) =35359120940a(20) =137847221148a(21) =538346579034