[1] R. G. Busacker T. L. Saaty: Finite Graphs and Networks: An Introduction with Applications. McGraw-Hill Book Company, New York 1965. MR 0209176

[2] S. Marcus: Algebraic Linguistics; Analytical Models. Academic Press, New York 1967. MR 0225610 | Zbl 0174.02402

[3] L. Nebeský: Algebraic Properties of Trees. Karlova universita, Praha 1969. MR 0274210

[4] L. Nebeský: Left-right double trees. Discrete Mathematics 1 (1971), 73-81. MR 0297620

[5] P. Novák: Postscript. [3], 83-95.