a(n) = 2*(2^n-1)*3^(n-1).

Terms

a(0) =0a(1) =2a(2) =18a(3) =126a(4) =810a(5) =5022a(6) =30618a(7) =185166a(8) =1115370a(9) =6705342a(10) =40271418a(11) =241746606a(12) =1450833930a(13) =8706066462a(14) =52239587418a(15) =313447090446