a(n) = 2^(n-1)*(2^n+5).

Terms

a(0) =3a(1) =7a(2) =18a(3) =52a(4) =168a(5) =592a(6) =2208a(7) =8512a(8) =33408a(9) =132352a(10) =526848a(11) =2102272a(12) =8398848a(13) =33574912a(14) =134258688a(15) =536952832a(16) =2147647488a(17) =8590262272a(18) =34360393728a(19) =137440264192a(20) =549758435328