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

Terms

a(0) =1a(1) =4a(2) =15a(3) =56a(4) =212a(5) =816a(6) =3184a(7) =12544a(8) =49728a(9) =197888a(10) =789248a(11) =3151872a(12) =12596224a(13) =50360320a(14) =201388032a(15) =805437440a(16) =3221504000a(17) =12885491712a(18) =51540852736a(19) =206161051648a(20) =824639225856