Sum is the state that two sets are separated, and the two sets are independent, so it is generally a problem of evaluation domain.

Multi-purpose, such as adding in (1, 2) and (2, 3) respectively means adding in these two sets, but f( 1.5) may be greater than f(2.5).

When two sets are merged into a large set, for example, increasing on (1, 2)∩(2, 3) and f( 1.5) must be less than f(2.5).