Sentence:  Spectral <WSD>graph</WSD> theory has a long history. In the early days, matrix theory and linear algebra were used to analyze adjacency matrices of <WSD>graphs</WSD>. Algebraic methods are especially effective in treating <WSD>graphs</WSD> which are regular and symmetric. 
Meaning of the word marked with <WSD> tags:  An ordered pair of sets (V,E), where the elements of V are called "vertices" or "nodes" and E is a set of pairs (called "edges") of elements of V;  a set of "vertices" (or "nodes") together with a set of "edges" that connect (some of) the vertices.