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

Terms

a(0) =0a(1) =1a(2) =4a(3) =17a(4) =70a(5) =282a(6) =1136a(7) =4583a(8) =18457a(9) =74131a(10) =296945a(11) =1190344a(12) =4773949a(13) =19145523a(14) =76751393a(15) =307503585a(16) =1231182078a(17) =4929075135a(18) =19736248104a(19) =79031964989a(20) =316476142454