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

Terms

a(0) =1a(1) =3a(2) =11a(3) =41a(4) =149a(5) =527a(6) =1823a(7) =6197a(8) =20777a(9) =68891a(10) =226355a(11) =738113a(12) =2391485a(13) =7705895a(14) =24712007a(15) =78918989a(16) =251105873a(17) =796364339a(18) =2518233179a(19) =7942120025a(20) =24988621541a(21) =78452649023a(22) =245818300271a(23) =768835960421