a(n) = 7*4^n.

Terms

a(0) =7a(1) =28a(2) =112a(3) =448a(4) =1792a(5) =7168a(6) =28672a(7) =114688a(8) =458752a(9) =1835008a(10) =7340032a(11) =29360128a(12) =117440512a(13) =469762048a(14) =1879048192a(15) =7516192768a(16) =30064771072a(17) =120259084288a(18) =481036337152