a(n) = Sum_{d|n} d * 4^(d-1).

Terms

a(0) =1a(1) =9a(2) =49a(3) =265a(4) =1281a(5) =6201a(6) =28673a(7) =131337a(8) =589873a(9) =2622729a(10) =11534337a(11) =50338105a(12) =218103809a(13) =939552777a(14) =4026533169a(15) =17180000521a(16) =73014444033a(17) =309238241337