Bang-Jensen, Jørgen; Gutin, Gregory (2018), "Basic Terminology, Notation and Results", in Bang-Jensen, Jørgen; Gutin, Gregory (eds.), Classes of Directed Graphs, Springer Monographs in Mathematics, Springer International Publishing, pp. 1–34, doi:10.1007/978-3-319-71840-8_1, ISBN978-3-319-71839-2; see p. 17
Bang-Jensen, Jørgen; Gutin, Gregory (2018), "Basic Terminology, Notation and Results", in Bang-Jensen, Jørgen; Gutin, Gregory (eds.), Classes of Directed Graphs, Springer Monographs in Mathematics, Springer International Publishing, pp. 1–34, doi:10.1007/978-3-319-71840-8_1, ISBN978-3-319-71839-2; see p. 17