a(n) = 19*3^n.

Terms

a(0) =19a(1) =57a(2) =171a(3) =513a(4) =1539a(5) =4617a(6) =13851a(7) =41553a(8) =124659a(9) =373977a(10) =1121931a(11) =3365793a(12) =10097379a(13) =30292137a(14) =90876411a(15) =272629233a(16) =817887699a(17) =2453663097a(18) =7360989291a(19) =22082967873a(20) =66248903619a(21) =198746710857a(22) =596240132571