a(0)=1, a(1)=2; for n>1, a(n) = n*(2^n+4)/4.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =20a(5) =45a(6) =102a(7) =231a(8) =520a(9) =1161a(10) =2570a(11) =5643a(12) =12300a(13) =26637a(14) =57358a(15) =122895a(16) =262160a(17) =557073a(18) =1179666a(19) =2490387a(20) =5242900a(21) =11010069a(22) =23068694a(23) =48234519a(24) =100663320a(25) =209715225a(26) =436207642a(27) =905969691a(28) =1879048220a(29) =3892314141