Alternating Hamiltonian cycles.

*(English)*Zbl 0325.05114For natural numbers \(n\) and \(d\), let \(K_n(\Delta_c \leq d)\) denote a complete graph of order \(n\) whose edges are colored so that no vertex belongs to more than \(d\) edges of the same color, and where \(\Delta_c\) is the maximal degree in the subgraph formed by the edges of color \(c\). D. E. Daykin proved that if \(d=2\) and \(n \geq 6\), then every such graph contains an alternating hamiltonian cycle (i.e. a spanning cycle whose adjacent edges have different colors). The authors have extended this as follows. Theorem: If \(69d <n\), then every \(G=K_n(\Delta_c \leq d)\) contains an alternating hamiltonian cycle. In fact, it is stated that if \(69d<n\), then every \(G=K_n(\Delta_c \leq d)\) contains alternating cycles of length \(\ell\) for every \(\ell\), \(3 \leq \ell \leq n\). An analogous result is obtained as follows. Let \(\chi_v\) denote the number of colors appearing among the edges containing the vertex \(v\), and let \(K_n(\chi_v \geq \lambda)\) denote a complete graph of order \(n\) whose edges are colored so that each vertex is on at least \(\lambda\) edges of different color. Theorem: Every \(K_n(\chi_v \geq (7/8)n)\) contains an alternating hamiltonian cycle. Several related results and conjectures are also presented.

Reviewer: S.F.Kapoor

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\textit{B. Bollobás} and \textit{P. Erdős}, Isr. J. Math. 23, 126--131 (1976; Zbl 0325.05114)

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##### References:

[1] | D. E. Daykin,Graphs with cycles having adjacent lines different colours, to appear. · Zbl 0287.05105 |

[2] | P. Erdös and T. Gallai,On maximal paths and circuits of graphs, Acta Math. Acad. Sci. Hungar.10, (1959), 337–356. · Zbl 0090.39401 |

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