a(n) = (5*2^(2*n)+(-2)^(n+1))/3.

Terms

a(0) =1a(1) =8a(2) =24a(3) =112a(4) =416a(5) =1728a(6) =6784a(7) =27392a(8) =109056a(9) =437248a(10) =1746944a(11) =6991872a(12) =27959296a(13) =111853568a(14) =447381504a(15) =1789591552a(16) =7158235136a(17) =28633202688a(18) =114532286464a(19) =458130194432