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Combinatorics Difficulty 8.8 Shortlist Prove it United States

At a certain orphanage, every pair of orphans are either friends or enemies of each other. For every three of an orphan's friends, an even number of pairs of them are enemies. Prove that it's possible to assign each orphan two parents such that every pair of friends shares exactly one parent, but no pair of enemies does, and no three parents are in a love triangle (where each pair of them has a child).

Solution

Solution: Form a graph GG with the orphans as vertices and with an edge between two orphans if they are friends. Let A1,A2,A_1, A_2, \dots be the maximal cliques of GG; that is, every pair of orphans in AiA_i is friends, and every orphan outside AiA_i is an enemy of some orphan in AiA_i. We now have a sequence of lemmas.

Lemma 1. Any two maximal cliques intersect in at most one orphan.
Proof. Suppose for the sake of contradiction that AiA_i and AjA_j had intersection of size at least 2. We may choose orphans uAiu \in A_i and vAjv \in A_j such that uu is not friends with vv; in particular, this means that uAju \notin A_j and vAiv \notin A_i. Then, taking two orphans w,wAiAjw, w' \in A_i \cap A_j, notice that ww is friends with uu, vv, and ww' and that {u,w}\{u, w'\} and {v,w}\{v, w'\} are pairs of friends, but {u,v}\{u, v\} is not, contradicting the given. This proves the lemma. □

Lemma 2. Every orphan vv is in either 1 or 2 of the AiA_i.
Proof. No orphan is in none of the AiA_i, as any orphan forms a clique of size 1, which may be grown into a maximal clique. Suppose for the sake of contradiction that some orphan vv were in at least 3 maximal cliques. Without loss of generality, let these cliques include A1,A2A_1, A_2, and A3A_3, and choose friends v1A1,v2A2v_1 \in A_1, v_2 \in A_2, and v3A3v_3 \in A_3 of vv.
For iji \neq j, notice that viAjv_i \notin A_j, as AjA_j intersects AiA_i in at most one orphan, which is vv. Thus, v1,v2v_1, v_2, and v3v_3 are distinct. If, say, v1v_1 and v2v_2 are friends, then {v,v1,v2}\{v, v_1, v_2\} forms a clique, hence is contained in some maximal clique AA; then AA intersects both A1A_1 and A2A_2 in at least two orphans, implying by Lemma 1 that A1=A=A2A_1 = A = A_2, a contradiction. Thus, there are no friendships between v1,v2v_1, v_2, and v3v_3, which contradicts the given because vv is friends with each of them. □

Form a graph HH' with vertices PAP_A the maximal cliques of GG and with an edge between two vertices if the corresponding cliques intersect. From HH', form the graph HH by attaching a leaf P{A,v}P_{\{A,v\}} to the maximal clique AA for each orphan vv for which AA is the unique maximal clique containing vv. Assign a parent to each vertex of HH and assign orphans to parents in the following way.

* If an orphan vv lies in only one maximal clique AA, assign it to PAP_A and P{A,v}P_{\{A,v\}}.
* If an orphan vv lies in two maximal cliques AA and AA', assign it to PAP_A and PAP_{A'}.

This construction covers all cases by Lemma 2 and assigns each pair of parents at most one orphan by Lemma 1. By construction, if an orphan is assigned to two parents, there is an edge between those parents in HH. Further, two orphans share a parent if and only if they share a maximal clique, meaning that they are friends. Finally, if HH contained a love triangle, then we would have three maximal cliques A,A,A, A', and AA'' with non-empty pairwise intersections at v,v,vv, v', v''. Then {v,v,v}\{v, v', v''\} is a clique, hence lies in a maximal clique AA^* which intersects each of A,A,A, A', and AA'' with cardinality at least 2. By Lemma 1, this implies that A=A=A=AA^* = A = A' = A'', a contradiction. Thus, HH has no love triangles, completing the proof.

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