a(n) = A047098(n)/2.

Terms

a(0) =1a(1) =4a(2) =19a(3) =98a(4) =531a(5) =2974a(6) =17060a(7) =99658a(8) =590563a(9) =3540464a(10) =21430267a(11) =130771376a(12) =803538100a(13) =4967127736a(14) =30866224824a(15) =192696614730