a(n) = (18^n - (-6)^n)/24.

Terms

a(0) =0a(1) =1a(2) =12a(3) =252a(4) =4320a(5) =79056a(6) =1415232a(7) =25520832a(8) =459095040a(9) =8265390336a(10) =148766948352