E.g.f. (1-x)/(1-2x-3x^2+3x^3).

Terms

a(0) =1a(1) =1a(2) =10a(3) =60a(4) =768a(5) =9480a(6) =161280a(7) =2968560a(8) =65036160a(9) =1568004480a(10) =42507763200