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

Terms

a(0) =1a(1) =4a(2) =19a(3) =91a(4) =427a(5) =1963a(6) =8875a(7) =39595a(8) =174763a(9) =764587a(10) =3320491a(11) =14330539a(12) =61516459a(13) =262843051a(14) =1118481067a(15) =4742359723a(16) =20043180715a(17) =84467690155a(18) =355050629803