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A Hamiltonian graph (directed or undirected) is a graph that contains a Hamiltonian cycle, that is, a cycle that visits every vertex exactly once.

3 votes
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Does this graph contain at least two Hamiltonian cycles?

Here is a proof of Gordon's claim. We will prove something slightly stronger. Claim. Let $G$ be a $d$-regular graph with $d$ odd. Then for every $e \in E(G)$, there is an even number of Hamiltoni …
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3 votes

How to efficiently find a Hamiltonian cycle in a graph whose closure is complete?

In the case that your graph satisfies the conditions of Ore's theorem (so that it's Ore closure is $K_n$ after 'one step'), there is an easy algorithm to find a Hamilton cycle. Arbitrarily arrange …
Tony Huynh's user avatar
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9 votes

What is the smallest uniquely hamiltonian graph with minimum degree at least 3?

I am not sure what the smallest such graph is, but since you also asked for more information on uniquely hamiltonian graphs with minimum degree $3$, Entringer and Swart proved the following nice theor …
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4 votes
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Hamiltonian cycle in $S_n$ with transpositions

Yes, $(S_n, E_n)$ contains a Hamiltonian cycle for every $n \geq 3$. This follows by the Steinhaus–Johnson–Trotter algorithm . The transpositions can even be chosen to be consecutive elements in the …
Tony Huynh's user avatar
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10 votes
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How many hamiltonian cycles can be removed from a complete directed graph before it becomes ...

I will rephrase your question slightly. Let $K_{n}^{*}$ be the directed graph with $n$ vertices and two oppositely directed edges for each pair of vertices. Your question is then the following. …
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6 votes

Efficient Hamiltonian cycle algorithms for graph classes

One class of graphs for which many NP-hard problems (including finding a Hamiltonian cycle) are easy (linear-time) are graphs of bounded tree-width. Indeed, by Courcelle's theorem any problem which c …
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5 votes
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Approximation of Hamiltonian cycles

Claim. For every $\rho \geq 1$, there is no polynomial $\rho$-approximation algorithm for $\texttt{MinHalfSimpCycle}$, unless P=NP. Proof. Let $G$ be an instance of the Travelling Salesman Problem (TS …
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