E.g.f. 1/(1-x-x^5).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =240a(6) =2160a(7) =20160a(8) =201600a(9) =2177280a(10) =29030400a(11) =439084800a(12) =7185024000a(13) =124540416000