Apparently equals A051133(n) - A005893(n) 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: A010936 A014806 A022615 * A061180 A125383 A126541
Adjacent sequences: A171072 A171073 A171074 * A171076 A171077 A171078
N. J. A. Sloane, Sep 06 2010