a(n) = 8*3^n - 1.

Terms

a(0) =7a(1) =23a(2) =71a(3) =215a(4) =647a(5) =1943a(6) =5831a(7) =17495a(8) =52487a(9) =157463a(10) =472391a(11) =1417175a(12) =4251527a(13) =12754583a(14) =38263751a(15) =114791255a(16) =344373767a(17) =1033121303a(18) =3099363911a(19) =9298091735a(20) =27894275207a(21) =83682825623a(22) =251048476871a(23) =753145430615