For these basic definitions, see Jensen & Toft (1995), pp. 1–2. Jensen, Tommy R.; Toft, Bjarne (1995), Graph coloring problems, Wiley-Interscience Series in Discrete Mathematics and Optimization, New York: John Wiley & Sons Inc., Theorem 1, pp. 2–3, ISBN0-471-02865-7, MR1304254.
Jensen & Toft (1995), p. 5. Jensen, Tommy R.; Toft, Bjarne (1995), Graph coloring problems, Wiley-Interscience Series in Discrete Mathematics and Optimization, New York: John Wiley & Sons Inc., Theorem 1, pp. 2–3, ISBN0-471-02865-7, MR1304254.
Jensen & Toft (1995), Theorem 1, p. 2. Jensen, Tommy R.; Toft, Bjarne (1995), Graph coloring problems, Wiley-Interscience Series in Discrete Mathematics and Optimization, New York: John Wiley & Sons Inc., Theorem 1, pp. 2–3, ISBN0-471-02865-7, MR1304254.
Lake (1975), p. 171: "It is straightforward to prove [the De Bruijn–Erdős theorem] using the compactness theorem for first-order logic" Lake, John (1975), "A generalization of a theorem of de Bruijn and Erdős on the chromatic number of infinite graphs", Journal of Combinatorial Theory, Series B, 18 (2): 170–174, doi:10.1016/0095-8956(75)90044-1, MR0360335.
Lake (1975), p. 171: "It is straightforward to prove [the De Bruijn–Erdős theorem] using the compactness theorem for first-order logic" Lake, John (1975), "A generalization of a theorem of de Bruijn and Erdős on the chromatic number of infinite graphs", Journal of Combinatorial Theory, Series B, 18 (2): 170–174, doi:10.1016/0095-8956(75)90044-1, MR0360335.