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The Tutte polynomial, also called the dichromate or the Tutte–Whitney polynomial, is a polynomial in two variables which plays an important role in graph theory. It is defined for every undirected graph and contains information about how the graph is connected. It is denoted by . The importance of this polynomial stems from the information it contains about ''G''. Though originally studied in algebraic graph theory as a generalisation of counting problems related to graph coloring and nowhere-zero flow, it contains several famous other specialisations from other sciences such as the Jones polynomial from knot theory and the partition functions of the Potts model from statistical physics. It is also the source of several central computational problems in theoretical computer science. The Tutte polynomial has several equivalent definitions. It is equivalent to Whitney’s rank polynomial, Tutte’s own dichromatic polynomial and Fortuin–Kasteleyn’s random cluster model under simple transformations. It is essentially a generating function for the number of edge sets of a given size and connected components, with immediate generalisations to matroids. It is also the most general graph invariant that can be defined by a deletion–contraction recurrence. Several textbooks about graph theory and matroid theory devote entire chapters to it.〔.〕〔.〕〔.〕 ==Definitions== Definition. For an undirected graph one may define the Tutte polynomial as The same definition can be given using slightly different notation by letting denote the rank of the graph . Then the Whitney rank generating function is defined as : The two functions are equivalent under a simple change of variables: : Tutte’s dichromatic polynomial is the result of another simple transformation: : Tutte’s original definition of is equivalent but less easily stated. For connected ''G'' we set : where denotes the number of spanning trees of “internal activity ''i'' and external activity ''j''.” A third definition uses a deletion–contraction recurrence. The edge contraction ''G''/''uv'' of graph ''G'' is the graph obtained by merging the vertices ''u'' and ''v'' and removing the edge ''uv''. We write ''G'' − ''uv'' for the graph where the edge ''uv'' is merely removed. Then the Tutte polynomial is defined by the recurrence relation : if ''e'' is neither a loop nor a bridge, with base case : if ''G'' contains ''i'' bridges and ''j'' loops and no other edges. Especially, if ''G'' contains no edges. The random cluster model from statistical mechanics due to provides yet another equivalent definition.〔.〕 The polynomial : is equivalent to under the transformation〔.〕 : 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tutte polynomial」の詳細全文を読む スポンサード リンク
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