Bisection of A000031.

Terms

a(0) =2a(1) =4a(2) =8a(3) =20a(4) =60a(5) =188a(6) =632a(7) =2192a(8) =7712a(9) =27596a(10) =99880a(11) =364724a(12) =1342184a(13) =4971068a(14) =18512792a(15) =69273668a(16) =260301176a(17) =981706832a(18) =3714566312a(19) =14096303344a(20) =53634713552a(21) =204560302844a(22) =781874936816