E.g.f. (1+x-x^2)^2/(1-x)^2.

Terms

a(0) =1a(1) =4a(2) =12a(3) =36a(4) =168a(5) =960a(6) =6480a(7) =50400a(8) =443520a(9) =4354560a(10) =47174400a(11) =558835200a(12) =7185024000a(13) =99632332800