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2. N. Biggs, Algebraic Graph Theory (Cambridge Univ. Press, Cambridge).
3. σ-Polynomials and graph colouring;Korfhage;J. Combin. Theory Ser. B,1978
4. Some chromatically unique graphs with seven points;Frucht;Ars Combin.,1983
5. An introduction of chromatic polynomials;Read;J. Combin. Theory,1968