A simple proof under.
All items into three parts: the front piece after piece, there are invisible. Obviously a term only and must be present in one part of the three. With this upper bound .
Obviously some of which it is unreasonable.
Antecedent rule for the space-time, the rules after piece is empty, you have to . So minus 2 times it is repeated subtracted plus 1 (representative of the front member, the rear member are empty rule).
Q.E.D.
Conclusion: The combination of math is really important ah.