a(0)=0; for n>0, a(n) = 4^n - 2*3^(n-1).

Terms

a(0) =0a(1) =2a(2) =10a(3) =46a(4) =202a(5) =862a(6) =3610a(7) =14926a(8) =61162a(9) =249022a(10) =1009210a(11) =4076206a(12) =16422922a(13) =66045982a(14) =265246810a(15) =1064175886a(16) =4266269482a(17) =17093775742a(18) =68461196410a(19) =274103065966