Value of an urn with n balls of type -1 and n+1 balls of type +1.

Terms

a(0) =1a(1) =4a(2) =15a(3) =58a(4) =226a(5) =882a(6) =3457a(7) =13606a(8) =53683a(9) =212090a(10) =838484a(11) =3319596a(12) =13159676a(13) =52220801a(14) =207374051a(15) =823906473a(16) =3274464556a(17) =13019414133a(18) =51790661881a(19) =206112818880a(20) =820583696869