a(n) = (3^(n+1) - 2n^2 + 4n + 5) / 8.

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =29a(5) =88a(6) =268a(7) =812a(8) =2449a(9) =7366a(10) =22124a(11) =66406a(12) =199261a(13) =597836a(14) =1793572a(15) =5380792a(16) =16142465a(17) =48427498a(18) =145282612a(19) =435847970a(20) =1307544061a(21) =3922632352a(22) =11767897244a(23) =35303691940a(24) =105911076049a(25) =317733228398a(26) =953199685468