Expansion of g.f.: x^2*(1-2*x) / (1-3*x-3*x^2+2*x^3).

Terms

a(0) =0a(1) =1a(2) =1a(3) =6a(4) =19a(5) =73a(6) =264a(7) =973a(8) =3565a(9) =13086a(10) =48007a(11) =176149a(12) =646296a(13) =2371321a(14) =8700553a(15) =31923030a(16) =117128107a(17) =429752305a(18) =1576795176a(19) =5785386229a(20) =21227039605a(21) =77883687150a(22) =285761407807