A009561 Index of central binomial coefficient C(2n,n) within A006987.
1, 4, 13, 28, 61, 119, 226, 423, 790, 1472, 2764, 5224, 9956, 19112, 36923, 71703, 139835, 273625, 536860, 1055588, 2079066, 4100573, 8096789, 16002500, 31652194, 62648225, 124068499, 245826760
Offset: 2