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Given a function: from a set X (the domain) to a set Y (the codomain), the graph of the function is the set [4] = {(, ()):}, which is a subset of the Cartesian product.In the definition of a function in terms of set theory, it is common to identify a function with its graph, although, formally, a function is formed by the triple consisting of its domain, its codomain and its graph.
Let G be a k-regular graph with 2n nodes. If k is sufficiently large, it is known that G has to be 1-factorable: If k = 2n − 1, then G is the complete graph K 2n, and hence 1-factorable (see above). If k = 2n − 2, then G can be constructed by removing a perfect matching from K 2n. Again, G is 1-factorable.
A graph with three vertices and three edges. A graph (sometimes called an undirected graph to distinguish it from a directed graph, or a simple graph to distinguish it from a multigraph) [4] [5] is a pair G = (V, E), where V is a set whose elements are called vertices (singular: vertex), and E is a set of unordered pairs {,} of vertices, whose elements are called edges (sometimes links or lines).
Then by the Tutte theorem G contains a perfect matching. Let G i be a component with an odd number of vertices in the graph induced by the vertex set V − U. Let V i denote the vertices of G i and let m i denote the number of edges of G with one vertex in V i and one vertex in U. By a simple double counting argument we have that
For a graph , the deck of G, denoted (), is the multiset of isomorphism classes of all vertex-deleted subgraphs of . Each graph in () is called a card. Two graphs that have the same deck are said to be hypomorphic. With these definitions, the conjecture can be stated as:
A family F of graphs or hypergraphs is defined to have the Erdős–Pósa property if there exists a function : such that for every (hyper-)graph G and every integer k one of the following is true: G contains k vertex-disjoint subgraphs each isomorphic to a graph in F ; or
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Let G be a simple plane graph with n vertices; we may add edges if necessary so that G is a maximally plane graph. If n < 3, the result is trivial. If n ≥ 3, then all faces of G must be triangles, as we could add an edge into any face with more sides while preserving planarity, contradicting the assumption of maximal planarity.