Algebraicgraph theory is a branch ofmathematics in whichalgebraic methods are applied to problems aboutgraphs. This is in contrast togeometric,combinatoric, oralgorithmic approaches. There are three main branches of algebraic graph theory, involving the use oflinear algebra, the use ofgroup theory, and the study ofgraph invariants.
The first branch of algebraic graph theory involves the study of graphs in connection withlinear algebra. Especially, it studies thespectrum of theadjacency matrix, or theLaplacian matrix of a graph (this part of algebraic graph theory is also calledspectral graph theory). For thePetersen graph, for example, the spectrum of the adjacency matrix is (−2, −2, −2, −2, 1, 1, 1, 1, 1, 3). Several theorems relate properties of the spectrum to othergraph properties. As a simple example, aconnected graph withdiameterD will have at leastD+1 distinct values in its spectrum.[1]Aspects of graph spectra have been used in analysing thesynchronizability ofnetworks.
The second branch of algebraic graph theory involves the study of graphs in connection togroup theory, particularlyautomorphism groups andgeometric group theory. The focus is placed on various families of graphs based onsymmetry (such assymmetric graphs,vertex-transitive graphs,edge-transitive graphs,distance-transitive graphs,distance-regular graphs, andstrongly regular graphs), and on the inclusion relationships between these families. Certain of such categories of graphs are sparse enough thatlists of graphs can be drawn up. ByFrucht's theorem, allgroups can be represented as the automorphism group of a connected graph (indeed, of acubic graph).[2] Another connection with group theory is that, given any group, symmetrical graphs known asCayley graphs can be generated, and these have properties related to the structure of the group.[1]
This second branch of algebraic graph theory is related to the first, since the symmetry properties of a graph are reflected in its spectrum. In particular, the spectrum of a highly symmetrical graph, such as the Petersen graph, has few distinct values[1] (the Petersen graph has 3, which is the minimum possible, given its diameter). For Cayley graphs, the spectrum can be related directly to the structure of the group, in particular to itsirreducible characters.[1][3]
Finally, the third branch of algebraic graph theory concerns algebraic properties ofinvariants of graphs, and especially thechromatic polynomial, theTutte polynomial andknot invariants. The chromatic polynomial of a graph, for example, counts the number of its propervertex colorings. For the Petersen graph, this polynomial is.[1] In particular, this means that the Petersen graph cannot be properly colored with one or two colors, but can be colored in 120 different ways with 3 colors. Much work in this area of algebraic graph theory was motivated by attempts to prove thefour color theorem. However, there are still manyopen problems, such as characterizing graphs which have the same chromatic polynomial, and determining which polynomials are chromatic.