Sequence with Hankel transform equal to 2^floor(n^2/2).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =112a(6) =560a(7) =2888a(8) =15136a(9) =80160a(10) =427968a(11) =2300736a(12) =12445440a(13) =67702272a(14) =370205184a(15) =2033976960a(16) =11224014336a(17) =62186741248a(18) =345825348608