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

Terms

a(0) =0a(1) =1a(2) =4a(3) =14a(4) =47a(5) =153a(6) =486a(7) =1516a(8) =4669a(9) =14255a(10) =43268a(11) =130818a(12) =394491a(13) =1187557a(14) =3570850a(15) =10728920a(16) =32219513a(17) =96724059a(18) =290303232a(19) =871171822a(20) =2614039735a(21) =7843167761a(22) =23531600414a(23) =70598995524a(24) =211805375157a(25) =635432902663