a(n) = A026520(2n).

Terms

a(0) =1a(1) =2a(2) =8a(3) =38a(4) =196a(5) =1052a(6) =5774a(7) =32146a(8) =180772a(9) =1024256a(10) =5837908a(11) =33433996a(12) =192239854a(13) =1109049320a(14) =6416509142a(15) =37215072638a(16) =216309089956