Spanning tree | Theorems in graph theory | Algebraic graph theory

Kirchhoff's theorem

In the mathematical field of graph theory, Kirchhoff's theorem or Kirchhoff's matrix tree theorem named after Gustav Kirchhoff is a theorem about the number of spanning trees in a graph, showing that this number can be computed in polynomial time from the determinant of a submatrix of the Laplacian matrix of the graph; specifically, the number is equal to any cofactor of the Laplacian matrix. Kirchhoff's theorem is a generalization of Cayley's formula which provides the number of spanning trees in a complete graph. Kirchhoff's theorem relies on the notion of the Laplacian matrix of a graph that is equal to the difference between the graph's degree matrix (a diagonal matrix with vertex degrees on the diagonals) and its adjacency matrix (a (0,1)-matrix with 1's at places corresponding to entries where the vertices are adjacent and 0's otherwise). For a given connected graph G with n labeled vertices, let λ1, λ2, ..., λn−1 be the non-zero eigenvalues of its Laplacian matrix. Then the number of spanning trees of G is (Wikipedia).

Kirchhoff's theorem
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Graph (discrete mathematics) | Characteristic polynomial | Indeterminate (variable) | Monomial | List of graph theory topics | Homogeneous polynomial | Graphic matroid | Multigraph | Prüfer sequence | Laplacian matrix | Minimum spanning tree | Determinant | Cayley's formula | Incidence matrix | Graph theory | Adjacency matrix | Mathematics | Vertex (graph theory) | Complete graph | Diamond graph | Degree matrix | Loop (graph theory) | Component (graph theory) | Markov chain tree theorem | Regular matroid