2n+5^n-3^n.

Terms

a(0) =0a(1) =4a(2) =20a(3) =104a(4) =552a(5) =2892a(6) =14908a(7) =75952a(8) =384080a(9) =1933460a(10) =9706596a(11) =48651000a(12) =243609208a(13) =1219108828a(14) =6098732684a(15) =30503229248a(16) =152544843936a(17) =762810312996