a(n) = (8*3^n - 5*0^n)/3.

Terms

a(0) =1a(1) =8a(2) =24a(3) =72a(4) =216a(5) =648a(6) =1944a(7) =5832a(8) =17496a(9) =52488a(10) =157464a(11) =472392a(12) =1417176a(13) =4251528a(14) =12754584a(15) =38263752a(16) =114791256a(17) =344373768a(18) =1033121304a(19) =3099363912a(20) =9298091736a(21) =27894275208a(22) =83682825624a(23) =251048476872a(24) =753145430616