a(n) = 3^n - 2*n - 1.

Terms

a(0) =0a(1) =0a(2) =4a(3) =20a(4) =72a(5) =232a(6) =716a(7) =2172a(8) =6544a(9) =19664a(10) =59028a(11) =177124a(12) =531416a(13) =1594296a(14) =4782940a(15) =14348876a(16) =43046688a(17) =129140128a(18) =387420452a(19) =1162261428a(20) =3486784360a(21) =10460353160a(22) =31381059564a(23) =94143178780a(24) =282429536432a(25) =847288609392