In graph theory, the Lovász conjecture (1970) is a classical problem on Hamiltonian paths in graphs. It says:

Every finite connected vertex-transitive graph contains a Hamiltonian path.

The original article of Lovász stated the result in the opposite, but this version became standard. In 1996 Babai published a conjecture sharply contradicting this conjecture^[1], but both conjectures remain widely open. It is not even known if a single counterexample would necessarily lead to a series of counterexamples.

Contents 1 Historical remarks 2 Variants of the Lovász conjecture 2.1 Hamiltonian cycle 2.2 Cayley graphs 2.3 Directed Cayley graph 3 Special cases 4 General groups 5 References

[edit] Historical remarks

The problem of finding Hamiltonian paths in highly symmetric graphs is quite old. As Knuth describes it in the forthcoming 4-th volume of The Art of Computer Programming^[2], the problem originated in British campanology (bell-ringing). Such Hamiltonian paths and cycles are also closely connected to Gray codes. In each case the constructions are explicit.

[edit] Variants of the Lovász conjecture

[edit] Hamiltonian cycle

Another version of Lovász conjecture states that

Every finite connected vertex-transitive graph contains a Hamiltonian cycle except the five known counterexamples.

There exists 5 examples of vertex-transitive graph with no Hamiltonian cycles (but with Hamiltonian paths) : the complete graph K₂, the Petersen graph, the Coxeter graph and two graphs derived from the Petersen and Coxeter graphs by replacing each vertex with a triangle.^[3]

[edit] Cayley graphs

None of the 5 vertex-transitive graphs with no Hamiltonian cycles is a Cayley graph, therefore that leads to a weaker version of the conjecture :

Every finite connected Cayley graph contains a Hamiltonian cycle.

The advantage of the Cayley graph formulation is that such graphs correspond to a finite group G and a generating set S. Thus one can ask for which G and S the conjecture holds rather than attack it in full generality.

[edit] Directed Cayley graph

For directed Cayley graphs (digraphs) the Lovász conjecture is false. Various counterexamples were obtained by R.A. Rankin. Still, many of the below results hold in this restrictive setting.

[edit] Special cases

The Lovász conjecture on Cayley is straightforward for abelian groups. It was proved in 1986 to hold for p-groups by D. Witte. It is open even for dihedral groups, although for special sets of generators some progress has been made.

When group G = S_n is a symmetric group, the are many attractive generating sets. For example, Lovász conjecture holds in the following cases of generating sets:

• (long cycle and a transposition).
• (Coxeter generators).
• any set of transpositions corresponding to a labelled tree on {1,2,..,n}.
• 

[edit] General groups

For general finite groups, only a few results are known:

• S = {a,b},(ab)² = 1 (R.A. Rankin generators)
• S = {a,b,c},a² = b² = c² = [a,b] = 1 (Rapaport-Strasser generators)
• S = {a,b,c},a² = 1,c = a ^{− 1}ba (Pak-Radoičić generators^[4])
• S = {a,b},a² = b^s = (ab)³ = 1, where (here we have (2,s,3)-presentation, Glover-Marušič theorem^[5]).

Finally, it is known that for every finite group G there exists a generating set of size at most log₂ | G | such that the corresponding Cayley graph is Hamiltonian (Pak-Radoičić). This result is based on classification of finite simple groups.

The Lovász conjecture was also established for random generating sets of size Ω(log⁵ | G | ).^[6]

[edit] References