a(n) = A026520(2n+1).

Terms

a(0) =1a(1) =4a(2) =20a(3) =104a(4) =556a(5) =3032a(6) =16778a(7) =93872a(8) =529684a(9) =3008864a(10) =17184188a(11) =98577712a(12) =567591142a(13) =3278348608a(14) =18986482250a(15) =110217131168a(16) =641125473092