a(n) = 2^n*(4*3^n-1).

Terms

a(0) =3a(1) =22a(2) =140a(3) =856a(4) =5168a(5) =31072a(6) =186560a(7) =1119616a(8) =6718208a(9) =40310272a(10) =241863680a(11) =1451186176a(12) =8707125248a(13) =52242767872a(14) =313456640000