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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.

5 votes
1 answer
277 views

Infinitely many counterexamples to Nash-Williams's conjecture about hamiltonicity?

Question from 2013 gives one counterexample to Nash-Williams's conjecture about hamiltonicity of dense digraphs. Later, we found tens of counterexamples on more than 30 vertices and believe there are …
2 votes
0 answers
116 views

Two more counterexamples to a conjecture from 1975 about hamiltonicity of digraphs

Question from 2013 gives one counterexample to Nash-Williams's conjecture 1975 about hamiltonicity of dense digraphs. In the linked answer, @LouisD "reverse engineered" the counterexample and pointed …
16 votes
1 answer
696 views

A counterexample to a conjecture of Nash-Williams about hamiltonicity of digraphs?

Maybe I am missing something, but found potential counterexample to a conjecture of Nash-Williams. According to HAMILTONIAN DEGREE SEQUENCES IN DIGRAPHS The outdegree and indegree sequences of digr …
1 vote
1 answer
178 views

Is Hamiltonian cycle fixed parameter tractable with parameter clique cover?

Let $G$ be connected simple graph. Clique cover of graph $G$ is partition of the vertices of $G$ into $k$ disjoint cliques $D'_i$. Given $G$ and $k$-clique cover, can we solve Hamiltonian cycle in …
1 vote
1 answer
78 views

Graph gadget related to uniquely hamiltionian regular graphs (question #2)

Related to uniquely hamiltionian graphs. For natural numbers $a,b$ define $(a,b)$ gadget $G$: $G$ is finite simple graph. Two vertices $u,v$ are of degree $b$ and the rest of the vertices are of degre …
5 votes
0 answers
93 views

Graph gadget related to uniquely hamiltionian regular graphs

A graph is uniquely hamiltonian if it has exactly one hamiltonian cycle. According to a conjecture there are no $r$-regular uniquely hamiltonian graphs for $r > 2$ and of special interest is the case …
3 votes
1 answer
216 views

Regular graphs with $a$ and $b$ Hamiltonian edges

Special case of this question. Let $G$ be $r$-regular Hamiltonian graph. An $a$ edge is an edge which is on every Hamiltonian cycle. A $b$ edge is an edge which is on no Hamiltonian cycle. $a(G)$ an …