Marcu, Dănuţ. “Free Sets Associated to a Finite Oriented Graph”. Anal. Numér. Théor. Approx. 10, no. 1 (February 1, 1981): 57–74. Accessed September 4, 2025. https://www.ictp.acad.ro/jnaat/journal/article/view/1981-vol10-no1-art7.