G.f.: Sum_{n>=0} x^n * Product_{k=0..n-1} (1 + k*x + x^2).

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =6a(5) =13a(6) =33a(7) =85a(8) =234a(9) =675a(10) =2032a(11) =6367a(12) =20677a(13) =69442a(14) =240529a(15) =857634a(16) =3141970a(17) =11808611a(18) =45464065a(19) =179088744a(20) =720947705a(21) =2962994169a(22) =12420658682a(23) =53061133078a(24) =230828047288