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

Terms

a(0) =1a(1) =7a(2) =36a(3) =164a(4) =704a(5) =2928a(6) =11968a(7) =48448a(8) =195072a(9) =783104a(10) =3138560a(11) =12567552a(12) =50298880a(13) =201256960a(14) =805158912a(15) =3220914176a(16) =12884246528a(17) =51538231296a(18) =206155546624a(19) =824627691520