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This two-graph is a regular two-graph since each pair of distinct vertices appears together in exactly two triples. Given a simple graph G = (V,E), the set of triples of the vertex set V whose induced subgraph has an odd number of edges forms a two-graph on the set V. Every two-graph can be represented in this way. [1]
When G″ ⊂ G and there exists an isomorphism between the sub-graph G″ and a graph G′, this mapping represents an appearance of G′ in G. The number of appearances of graph G′ in G is called the frequency F G of G′ in G. A graph is called recurrent (or frequent) in G when its frequency F G (G′) is above a predefined threshold or ...
Although the enharmonic key of A-flat major is preferred because A-flat major has only four flats as opposed to G-sharp major's eight sharps (including the F), G-sharp major appears as a secondary key area in several works in sharp keys, most notably in the Prelude and Fugue in C-sharp major from Johann Sebastian Bach's The Well-Tempered Clavier, Book 1.
Two graphs are isomorphic if there is an isomorphism between them; see isomorphism. isomorphism A graph isomorphism is a one-to-one incidence preserving correspondence of the vertices and edges of one graph to the vertices and edges of another graph. Two graphs related in this way are said to be isomorphic. isoperimetric See expansion. isthmus
A set of graphs isomorphic to each other is called an isomorphism class of graphs. The question of whether graph isomorphism can be determined in polynomial time is a major unsolved problem in computer science, known as the graph isomorphism problem. [1] [2] The two graphs shown below are isomorphic, despite their different looking drawings.
Clique-sums have a close connection with treewidth: If two graphs have treewidth at most k, so does their k-clique-sum.Every tree is the 1-clique-sum of its edges. Every series–parallel graph, or more generally every graph with treewidth at most two, may be formed as a 2-clique-sum of triangles.
Two graphs G and H are homomorphically equivalent if G → H and H → G. [4] The maps are not necessarily surjective nor injective. For instance, the complete bipartite graphs K 2,2 and K 3,3 are homomorphically equivalent: each map can be defined as taking the left (resp. right) half of the domain graph and mapping to just one vertex in the left (resp. right) half of the image graph.
A graph G is said to be k-constructible (or Hajós-k-constructible) when it formed in one of the following three ways: [1] The complete graph K k is k-constructible. Let G and H be any two k-constructible graphs. Then the graph formed by applying the Hajós construction to G and H is k-constructible.