Hamiltonian graph - Definition and Overview

In the mathematical field of graph theory, a Hamiltonian path is a path in a undirected graph which visits each vertex exactly once. Determining whether such a path exists is an NP-complete problem called the Hamiltonian path problem.

Hamiltonian paths and cycles are named after William Rowan Hamilton who invented the icosian game, now also known as Hamilton's puzzle, which involves finding a Hamiltonian cycle in the edge graph of the dodecahedron. Hamilton solved this problem using the icosian calculus, an algebraic structure based on roots of unity with many similarities to the quaternions (also invented by Hamilton). Unfortunately, this solution does not generalize to arbitrary graphs.

Definition

A Hamiltonian path or traceable path is a path that visits each vertex exactly once. A graph that contains a Hamiltonian path is called a traceable graph.

A Hamiltonian cycle, Hamiltonian circuit, vertex tour or graph cycle is a cycle that visits each vertex exactly once (excluding the start/end vertex). A graph that contains a Hamiltonian cycle is called a Hamiltonian graph.

Similar notions may be defined for directed graphs, where edges (arcs) of a path or a cycle are required to point in the same direction, i.e., connected tail-to-head.

Examples

• a complete graph with more than two vertices is hamiltonian
• every circle graph is hamiltonian
• every tournament has an odd number of Hamiltonian paths
• every platonic solid, considered as a graph, is hamiltonian

Notes

Any Hamiltonian cycle can be converted to a Hamiltonian path by removing one of its edges, but a Hamiltonian path can be extended to Hamiltonian cycle only if its endpoints are adjacent.

If an undirected graph G is hamiltonian then his line graph L(G) is hamiltonian too.

See also

• Eulerian path

External links

• Weisstein, Eric W., Hamiltonian Circuit (http://mathworld.wolfram.com/HamiltonianCircuit.html). Wolfram Research, 2003.

References

• DeLeon, Melissa, "A Study of Sufficient Conditions for Hamiltonian Cycles (http://www.rose-hulman.edu/mathjournal/2000/vol1-n1/paper4/v1n1-4pd.PDF)". Department of Mathematics and Computer Science, Seton Hall University, South Orange, New Jersey, United States of America. [PDF]
• Hamilton, William Rowan, "Memorandum respecting a new system of roots of unity". Philosophical Magazine, 12 1856
• Hamilton, William Rowan, "Account of the Icosian Calculus". Proceedings of the Royal Irish Academy, 6 1858