Apparently equals A051133(n) - A090288(n-1) after 1st term.
Note that A051133 yields the totals of all C(2n, n) ball sets and A005893 and A090288 are the respective totals of impossible combinations given the rules.
David Scambler, Just for fun, more tennis balls, Posting to the Sequence Fans Mailing List, Aug 25 2010.
Table of n, a(n) for n=1..7.
Sequence in context: A037566 A125857 A226312 * A287999 A001078 A299865
Adjacent sequences: A171073 A171074 A171075 * A171077 A171078 A171079
N. J. A. Sloane, Sep 06 2010