Organización de los Centros de Profesores.

In this subsection, we will follow on from the work of Grandi and Endriss (2013) using the integrity constraint setting of binary aggregation. This looks at classes of aggregation rules which guarantee feasible outcomes for every member of a set of constraints. We focus on their following lemma.

Lemma 3.9 (Lemma 3, Grandi and Endriss, 2013). Given a set of formulas L ⊆ L(Φ). If an aggregation ruleF guarantees π-feasible outcomes on π-rational profiles for all π ∈ L, thenF guarantees (V

π∈Lπ)-feasible outcomes on (

V

π∈Lπ)-rational

profiles.

In the following example from Chen and Endriss (2018) we see that the converse does not hold.

Example 7. Consider the following clauses as our rationality and feasibility con- straints: π1 = ¬ϕa∨ ¬ϕb and π2 = ϕa. We see from Grandi and Endriss (2013,

Corollary 31) (the single-constraint case of Corollary 3.8), that taking π1 as the

outcomes is when u > n₂. Similarly, when taking π2 as the constraint, we get

that the class of uniform quota rules which guaranteeπ2-feasible outcomes is when

u < n+ 1. From Lemma 3.9, we see that a uniform quota rule guarantee(π1∧π2)-

feasible outcomes on(π1∧π2)-rational profiles whenuabides byn+ 1> u > n₂.

However, we see thatπ1∧π2 ≡ϕa∧ ¬ϕb. It is clear that any non-trivial quota rule

will give(π1∧π2)-feasible outcomes on (π1∧π2)-rational profiles. Therefore, the

previous range n+ 1 > u > n₂ does not fully capture all of the aggregation rules which give(π1∧π2)-feasible outcomes on(π1∧π2)-rational profiles.

Moving away from Chen and Endriss’s example, we can look to prime implicates fora solution to this problem. There are two prime implicates of(π1∧π2), namely

¬ϕbandϕa. Thus, by applying the result from Grandi and Endriss (2013, Corollary

31) on both prime implicates, we get thatn+ 1> u >0. This captures all possible non-trivial quota rules that can lead to(π1∧π2)-feasible outcomes. 4

The previous example motivates Theorem 3.11. However, first we will prove a lemma in order to prove this theorem. Note that this lemma appears to be similar to the left-to-right direction of Theorem 3.7. However, this lemma is not a special case of Theorem 3.7.

Lemma 3.10. LetΓbe a formula,π be a prime implicate ofΓwithkliterals, and let

F be a quota rule. IfF guaranteesπ-feasible outcomes onΓ-rational profiles thenF

either has a trivial quota with respect toπ, or the quotas of the issues in Var(π)abide by: X ϕ∈Var(π−₎ q(ϕ) + X ϕ∈Var(π+₎ (n−q(ϕ) + 1)> n(k−1).

Proof. First, it is worth noting that when Γ is inconsistent, then there are no Γ- rational profiles. Therefore, the lemma is vacuously true. Thus we will assume that Γis consistent and that there exists someΓ-rational profiles.

We shall prove the lemma by contraposition. Therefore, we assume that F does not abide by the inequality for the quotas of the issues inπ, as well as there being no trivial quotas with respect to π. We want to show that there exists aΓ-rational profile without aπ-feasible outcome. Consider the following profile,B, depicted in Table 3.5.

In Table 3.5, we let π = Wk

i=1Li, whereLi is a literal which is equal to either ϕi

or ¬ϕi. We see each judgment agrees with one of the literals of π and disagrees

with the rest. In the ‘remaining issues’ column we see that every judgment in B

# of agents ϕ1 ϕ2 ϕ3 ... ϕk Remaining Issues:Var(Γ)\Var(π) `1 C W W ... W C `2 W C W ... W C `3 W W C ... W C ... `k W W W ... C C Outcome W W W ... W N/A

Table 3.5: ProfileB for the proof Lemma 3.10

aΓ-rational profile.

Suppose that a judgment supportingLi∧Vj∈[1,k]\{i}¬Lj from Table 3.5 is incon-

sistent withΓ(withi∈[1, k]). Therefore,Bcannot be aΓ-rational profile. Hence, we have that Γ∧Li ∧Vj∈[1,k]\{i}¬Lj ⊥, or Γ ¬(Li ∧Vj∈[1,k]\{i}¬Lj), which

is equivalent toΓ ¬Li∨

W

j∈[1,k]\{i}Lj. Therefore, ¬Li∨

W

j∈[1,k]\{i}Lj is an im-

plicate of Γ. As bothπ and¬Li∨W_j∈[1,k]\{i}Lj are implicates ofΓ, it is clear that

W

j∈[1,k]\{i}Lj must also be an implicate ofΓ. 1 However, this entails that π is not

a prime implicate of Γ, as the implicateW

j∈[1,k]\{i}Lj π, andπ 2 Wj∈[1,k]\{i}Lj.

Therefore, we can conclude thatB can be aΓ-rational profile.

Finally, we need to check that the profile depicted in Table 3.5 does not result in a

π-feasible outcome. Under the earlier made assumptions, there are no trivial quotas with respect to π. Furthermore, the quotas of the issues inπ do not abide by the inequality in the statement of the lemma. In the table, we let`j be equal to at most

q(ϕj)−1, ifLj is a positive literal in π. Otherwise,`j can be at most n−q(ϕj), if

Lj is the negative literal ofπ. Observe that the number of ‘wrong’ votes for an issue

ϕj (j∈[1, k]) in this profile is at leastn−q(ϕj) + 1ifLj is a positive literal inπ, or

at leastn−(n−q(ϕj))ifLj is a negative literal inπ.

From Table 3.5, we see that the total number of wrong votes for the issues inπ is P ϕj∈Var(π) n−`j 6 P ϕj∈Var(π−) q(ϕj) + P ϕj∈Var(π+) (n−q(ϕj) + 1). AsB is aΓ-rational

profile this means that there are at leastncorrect votes for issues inπ. Therefore, the total number of wrong votes for the issues in π can be at mostn(k−1)votes. Thus, we have that P

ϕj∈Var(π−)

q(ϕj) + P ϕj∈Var(π+)

(n−q(ϕj) + 1)6n(k−1), which is

consistent with our assumption.

B is a Γ-rational profile; however, the outcome is notπ-feasible. This is in part due to there being no trivial quota with respect toπ. Therefore, none of the literals in π will agree with the outcome, regardless of the profile submitted to the rule.

1_{This is more commonly known as the resolution rule, used to find (prime) implicates of a formula.}

Moreover, as eachϕj ∈Var(π)has received`j votes agreeing withLj, it is clear that

this is not enough for the outcome to agree with any of the literals ofπ. Therefore, we have found aΓ-rational profile without aΓ0-feasible outcome.

Now that we have shown the previous lemma, we can prove the following theorem.

Theorem 3.11. LetF be a quota rule, andΓbe a formula. F guaranteesΓ-feasible outcomes onΓ-rational profiles if and only if for all prime implicatesπ ofΓ,F guar- anteesπ-feasible outcomes onπ-rational profiles.

Proof. In the right-to-left direction, we let P be the set of prime implicates of Γ. ThusΓ is logically equivalent to V

π∈Pπ. Therefore, the right-to-left direction fol-

lows from Lemma 3.9, so left to prove is the left-to-right direction.

We assume thatF guaranteesΓ-feasible outcomes on Γ-rational profiles. We want to show that F also guarantees π-feasible outcomes on π-rational profiles, for all prime implicates ofΓ. We pick a prime implicateπofΓarbitrarily.

As F guarantees Γ-feasible outcomes on Γ-rational profiles, F also guarantees π- feasible outcomes onΓ-rational profiles. By Lemma 3.10 we see that this implies that F either has a trivial quota with respect toπ, or the quotas of the issues ofπ

abide by P

ϕ∈Var(π−₎

q(ϕ) + P

ϕ∈Var(π+₎

(n−q(ϕ) + 1)> n(k−1).

We see that these are the conditions in Theorem 30 by Grandi and Endriss (2013), for F to guarantee π-feasible outcomes on π-rational profiles. As π was chosen arbitrarily, we have that F guarantees π-feasible outcomes on π-rational profiles for all prime implicates ofΓ.