a(n) = (A001317(2n)-1)/4.

Terms

a(0) =0a(1) =1a(2) =4a(3) =21a(4) =64a(5) =321a(6) =1092a(7) =5461a(8) =16384a(9) =81921a(10) =278532a(11) =1392661a(12) =4210752a(13) =21053761a(14) =71582788a(15) =357913941a(16) =1073741824a(17) =5368709121a(18) =18253611012a(19) =91268055061a(20) =275951648832