a(n) = Sum_{d|n} (n/d) * 4^(n-d).

Terms

a(0) =1a(1) =9a(2) =49a(3) =289a(4) =1281a(5) =7041a(6) =28673a(7) =147969a(8) =602113a(9) =2951169a(10) =11534337a(11) =57876481a(12) =218103809a(13) =1056997377a(14) =4113563649a(15) =19394592769a(16) =73014444033a(17) =354385657857