E.g.f. : exp(6x)/(1-x).

Terms

a(0) =1a(1) =7a(2) =50a(3) =366a(4) =2760a(5) =21576a(6) =176112a(7) =1512720a(8) =13781376a(9) =134110080a(10) =1401566976a(11) =15780033792a(12) =191537187840