Weakly additive


In fair division, a topic in economics, a preference relation is weakly additive
if the following condition is met:
Every additive utility function is weakly-additive. However, additivity is applicable only to cardinal utility functions, while weak additivity is applicable to ordinal utility functions.
Weak additivity is often a realistic assumption when dividing up goods between claimants, and simplifies the mathematics of certain fair division problems considerably. Some procedures in fair division do not need the value of goods to be additive and only require weak additivity. In particular the adjusted winner procedure only requires weak additivity.

Cases where weak additivity fails

Case where the assumptions might fail would be either
The use of money as compensation can often turn real cases like these into situations where the weak additivity condition is satisfied even if the values are not exactly additive.
The value of a type of goods, e.g. chairs, dependent on having some of those goods already is called the marginal utility.