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A note on two circumference generalizations of Chvátal’s Hamiltonicity condition. (English) Zbl 1006.05036
Generalizing the well-known Chvátal’s Hamiltonicity condition B. Bollobás in his classical book on extremal graph theory asked: Let \(G\) be a \(2\)-connected graph of order \(n\) with vertex degrees \(d_1 \leq d_2 \leq \dots \leq d_n\). Suppose \(3 \leq c \leq n\) and \(d_i \leq i < c/2\) implies \(d_{n-i} \geq c-i\). Does it follow that for the circumference \(c(G)\) of \(G\) (the length of its longest cycle) \(c(G) \geq c\) holds? For \(c=3\) and \(c=4\) this follows directly from the \(2\)-connectivity of \(G\), however Häggkvist found counterexamples for any \(c \geq 7\). In the paper the author proves that for \(c=5\) the answer is affirmative while for \(c=6\) counterexamples are presented. An interesting generalization of Chvátal’s condition for any \(c\) is formulated and proved.
05C38 Paths and cycles
05C45 Eulerian and Hamiltonian graphs
05C07 Vertex degrees
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