Edge and vertex spaces

In the mathematical discipline of graph theory, the edge space and vertex space of an undirected graph are vector spaces defined in terms of the edge and vertex sets, respectively. These vector spaces make it possible to use techniques of linear algebra in studying the graph.

Definition

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Let   be a finite undirected graph. The vertex space   of G is the vector space over the finite field of two elements   of all functions  . Every element of   naturally corresponds the subset of V which assigns a 1 to its vertices. Also every subset of V is uniquely represented in   by its characteristic function. The edge space   is the  -vector space freely generated by the edge set E. The dimension of the vertex space is thus the number of vertices of the graph, while the dimension of the edge space is the number of edges.

These definitions can be made more explicit. For example, we can describe the edge space as follows:

  • elements are subsets of  , that is, as a set   is the power set of E
  • vector addition is defined as the symmetric difference:  
  • scalar multiplication is defined by:
    •  
    •  

The singleton subsets of E form a basis for  .

One can also think of   as the power set of V made into a vector space with similar vector addition and scalar multiplication as defined for  .

Properties

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The incidence matrix   for a graph   defines one possible linear transformation

 

between the edge space and the vertex space of  . The incidence matrix of  , as a linear transformation, maps each edge to its two incident vertices. Let   be the edge between   and   then

 

The cycle space and the cut space are subspaces of the edge space.

References

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  • Diestel, Reinhard (2005), Graph Theory (3rd ed.), Springer, ISBN 3-540-26182-6 (the electronic 3rd edition is freely available on author's site).

See also

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