2^n*(2^(n+1) - n - 1).

Terms

a(0) =1a(1) =4a(2) =20a(3) =96a(4) =432a(5) =1856a(6) =7744a(7) =31744a(8) =128768a(9) =519168a(10) =2085888a(11) =8364032a(12) =33501184a(13) =134103040a(14) =536625152a(15) =2146959360a(16) =8588820480a(17) =34357379072a(18) =137433972736a(19) =549745328128