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

Terms

a(0) =1a(1) =5a(2) =24a(3) =108a(4) =464a(5) =1936a(6) =7936a(7) =32192a(8) =129792a(9) =521472a(10) =2091008a(11) =8375296a(12) =33525760a(13) =134156288a(14) =536739840a(15) =2147205120a(16) =8589344768a(17) =34358493184a(18) =137436332032a(19) =549750308864