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Typical regular graphs are Hamiltonian

PDF p. 9: Section 1.5, citation to Robinson and Wormald [45]

Statement

Fix an integer d≥3. A uniformly random labelled simple d-regular graph on n vertices is Hamiltonian with high probability as n→∞ through feasible orders. Here a graph is d-regular when every vertex has degree d, and Hamiltonian means that it contains a cycle visiting every vertex exactly once. The uniform law gives equal probability to each simple d-regular graph on the labelled vertex set {1,…,n}; an order is feasible when at least one such graph exists. With high probability means that the probability tends to one along those orders.

Facts & Assumptions

[F1]

For each fixed integer d≥3, the probability that a uniformly sampled labelled simple d-regular graph is Hamiltonian tends to one as its order grows through feasible values. A Hamiltonian cycle is a spanning cyclic ordering with edges between consecutive vertices, including the last and first (Hamiltonicity of a uniform fixed-degree regular graph).

[F2]

The uniform random regular model assigns equal mass to the labelled simple d-regular graphs, and its high-probability limits are taken through feasible orders (Uniform random regular graph convention). The degree and regularity conventions agree with the graph notation (Adjacency, degrees, graph Laplacian, and spectral notation).

Proof

Given: A fixed integer d≥3 and the uniform labelled simple d-regular graph model at feasible orders.

Goal: Show that the graph is Hamiltonian with high probability as its order tends to infinity.

1.1givenF1F2

By [F2], the model and feasible-order limit in the statement are exactly those in [F1]; the stated cycle condition is the spanning cyclic ordering in [F1]. Thus [F1] applies to the Hamiltonian event in this model.

2.1F1F2step 1.1∎

The probability of that event therefore tends to one through feasible orders. By the high-probability convention in [F2], this is the claimed conclusion.

Source notes

The article attributes the random regular graph Hamiltonicity result to Robinson and Wormald in Section 1.5; the proof above uses the independently sourced prerequisite, not the article's assertion.

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Used by · 1

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