(2*4^n-(3^n-1))/2.

Terms

a(0) =1a(1) =3a(2) =12a(3) =51a(4) =216a(5) =903a(6) =3732a(7) =15291a(8) =62256a(9) =252303a(10) =1019052a(11) =4105731a(12) =16511496a(13) =66311703a(14) =266043972a(15) =1066567371a(16) =4273443936a(17) =17115299103a(18) =68525766492a(19) =274296776211