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

Terms

a(0) =2a(1) =9a(2) =24a(3) =90a(4) =258a(5) =1043a(6) =3440a(7) =13419a(8) =48850a(9) =187836a(10) =705444a(11) =2725099a(12) =10400614a(13) =40233015a(14) =155133856a(15) =601820876a(16) =2333606238a(17) =9079958260a(18) =35345263820a(19) =137876637843a(20) =538259060526