In graph theory, the tensor product G × H of graphs G and H is a graph such that
the vertex set of G × H is the Cartesian product V(G) × V(H); and
vertices (g,h) and math|(''g,h' ) are adjacent in G × H if and only if
g is adjacent to g' in G, and
h is adjacent to h' in H.
The tensor product is also called the direct product, Kronecker product, categorical product, cardinal product, relational product, weak direct product, or conjunction'''. As an operation on binary relations, the tensor product was introduced by Alfred North Whitehead and Bertrand Russell in their Principia Mathematica (1912). It is also equivalent to the Kronecker product of the adjacency matrices of the graphs.
The notation G × H is also (and formerly normally was) used to represent another construction known as the Cartesian product of graphs, but nowadays more commonly refers to the tensor product. The cross symbol shows visually the two edges resulting from the tensor product of two edges. This product should not be confused with the strong product of graphs.
The tensor product G × K2 is a bipartite graph, called the bipartite double cover of G. The bipartite double cover of the Petersen graph is the Desargues graph: K2 × G(5,2) = G(10,3). The bipartite double cover of a complete graph Kn is a crown graph (a complete bipartite graph Kn,n minus a perfect matching).
The tensor product of a complete graph with itself is the complement of a Rook's graph. Its vertices can be placed in an n-by-n grid, so that each vertex is adjacent to the vertices that are not in the same row or column of the grid.
The tensor product is the in the category of graphs and graph homomorphisms. That is, a homomorphism to G × H corresponds to a pair of homomorphisms to G and to H. In particular, a graph I admits a homomorphism into G × H if and only if it admits a homomorphism into G and into H.
To see that, in one direction, observe that a pair of homomorphisms f_G : I → G and f_H : I → H yields a homomorphism
In the other direction, a homomorphism f : I → G × H can be composed with the projections homomorphisms
to yield homomorphisms to G and to H.
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In graph theory, a branch of mathematics, a crown graph on 2n vertices is an undirected graph with two sets of vertices {u_1, u_2, ..., u_n} and {v_1, v_2, ..., v_n} and with an edge from u_i to v_j whenever i ≠ j. The crown graph can be viewed as a complete bipartite graph from which the edges of a perfect matching have been removed, as the bipartite double cover of a complete graph, as the tensor product K_n × K_2, as the complement of the Cartesian direct product of K_n and K_2, or as a bipartite Kneser graph H_n,1 representing the 1-item and (n – 1)-item subsets of an n-item set, with an edge between two subsets whenever one is contained in the other.
In graph theory, the Cartesian product G □ H of graphs G and H is a graph such that: the vertex set of G □ H is the Cartesian product V(G) × V(H); and two vertices (u,u' ) and (v,v' ) are adjacent in G □ H if and only if either u = v and u' is adjacent to v' in H, or u' = v' and u is adjacent to v in G. The Cartesian product of graphs is sometimes called the box product of graphs [Harary 1969]. The operation is associative, as the graphs (F □ G) □ H and F □ (G □ H) are naturally isomorphic.
In the mathematical field of graph theory, a graph homomorphism is a mapping between two graphs that respects their structure. More concretely, it is a function between the vertex sets of two graphs that maps adjacent vertices to adjacent vertices. Homomorphisms generalize various notions of graph colorings and allow the expression of an important class of constraint satisfaction problems, such as certain scheduling or frequency assignment problems.
We examine the connection of two graph parameters, the size of a minimum feedback arcs set and the acyclic disconnection. A feedback arc set of a directed graph is a subset of arcs such that after deletion the graph becomes acyclic. The acyclic disconnecti ...
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2022
We present polynomial families complete for the well-studied algebraic complexity classes VF, VBP, VP, and VNP. The polynomial families are based on the homomorphism polynomials studied in the recent works of Durand et al. (2014) and Mahajan et al. (2018). ...