2006IMO中国国家队选拔（第二十一届）.pdf

Mathematical Olympiad Express n n a b c d a b c d ad bc k k f x ( ) f 0 f 1 ( ) () n n f x f x x x ( ) ( ) 1 s 1 s s 2k x f x ∈Z f x ( ) ( ) m a b n an a,b ( ) bn A ∩ 1,2,,m { }