Binomial coefficient C(2n+1, n-1).

Terms

a(0) =1a(1) =5a(2) =21a(3) =84a(4) =330a(5) =1287a(6) =5005a(7) =19448a(8) =75582a(9) =293930a(10) =1144066a(11) =4457400a(12) =17383860a(13) =67863915a(14) =265182525a(15) =1037158320a(16) =4059928950a(17) =15905368710a(18) =62359143990a(19) =244662670200a(20) =960566918220