a(n) = (1/2)*A291417(n).

Terms

a(0) =2a(1) =9a(2) =38a(3) =161a(4) =680a(5) =2872a(6) =12128a(7) =51214a(8) =216264a(9) =913228a(10) =3856328a(11) =16284284a(12) =68764352a(13) =290374208a(14) =1226175744a(15) =5177825416a(16) =21864627616a(17) =92328709904a(18) =389880441696