The proportional rule is a division rule for solving bankruptcy problems. According to this rule, each claimant should receive an amount proportional to their claim. In the context of taxation, it corresponds to a proportional tax. [1]
There is a certain amount of money to divide, denoted by (=Estate or Endowment). There are nclaimants. Each claimant i has a claim denoted by . Usually, , that is, the estate is insufficient to satisfy all the claims.
The proportional rule says that each claimant i should receive , where r is a constant chosen such that . In other words, each agent gets .
Examples with two claimants:
Examples with three claimants:
The proportional rule has several characterizations. It is the only rule satisfying the following sets of axioms:
There is a variant called truncated-claims proportional rule, in which each claim larger than E is truncated to E, and then the proportional rule is activated. That is, it equals , where . The results are the same for the two-claimant problems above, but for the three-claimant problems we get:
The adjusted proportional rule [8] first gives, to each agent i, their minimal right, which is the amount not claimed by the other agents. Formally, . Note that implies .
Then, it revises the claim of agent i to , and the estate to . Note that that .
Finally, it activates the truncated-claims proportional rule, that is, it returns , where .
With two claimants, the revised claims are always equal, so the remainder is divided equally. Examples:
With three or more claimants, the revised claims may be different. In all the above three-claimant examples, the minimal rights are and thus the outcome is equal to TPROP, for example, .
Curiel, Maschler and Tijs [8] prove that the AP-rule returns the tau-value of the coalitional game associated with the bankruptcy problem.
The AP-rule is self-dual. In addition, it is the only rule satisfying the following properties:
In contrast, the truncated-proportional rule violates minimal-rights, and the proportional rule violates also Independence-of-irrelevant-claims. [8]