Index of central binomial coefficient C(2n,n) within A006987.

Terms

a(0) =1a(1) =4a(2) =13a(3) =28a(4) =61a(5) =119a(6) =226a(7) =423a(8) =790a(9) =1472a(10) =2764a(11) =5224a(12) =9956a(13) =19112a(14) =36923a(15) =71703a(16) =139835a(17) =273625a(18) =536860a(19) =1055588a(20) =2079066a(21) =4100573a(22) =8096789a(23) =16002500a(24) =31652194a(25) =62648225a(26) =124068499a(27) =245826760