æz ɐ klˈoʊsli ɹᵻlˈeɪɾᵻd kˈeɪs, ðə pˈɛɹən nˈʌmbɚɹ əvə ɡɹˈæf ɪz dᵻfˈaɪnd təbi ðə spˈɛktɹəl ɹˈeɪdɪəs ʌv ɪts ɐdʒˈeɪsənsi mˈeɪtɹɪks.
As a closely related case, the Perron number of a graph is defined to be the spectral radius of its adjacency matrix.