四边形不等式
不妨设 a < c a<c a<c,有:
w a , c + w a + 1 , c + 1 ≤ w a + 1 , c + w a , c + 1 ( 1 ) w_{a,c}+w_{a+1,c+1}\leq w_{a+1,c}+w_{a,c+1} (1) wa,c+wa+1,c+1≤wa+1,c+wa,c+1(1)
w a + 1 , c + w a + 2 , c + 1 ≤ w a + 1 , c + 1 + w a + 2 , c ( 2 ) w_{a+1,c}+w_{a+2,c+1}\leq w_{a+1,c+1}+w_{a+2,c}(2) wa+1,c+wa+2,c+1≤wa+1,c+1