On sparse hamiltonian 2-decompositions together with exact count of numerous Hamilton cycles

Abstract

Abstract We construct multigraphs of any large order with as few as only four 2-decompositions into Hamilton cycles or only two 2-decompositions into Hamilton paths. Nevertheless, some of those multigraphs are proved to have exponentially many Hamilton cycles (Hamilton paths). Two families of large simple graphs are constructed. Members in one class have exactly 16 hamiltonian pairs, in another class exactly four traceable pairs. These graphs also have exponentially many Hamilton cycles and Hamilton paths, respectively. The exact numbers of (Hamilton) cycles and paths are expressed in terms of Fibonacci-like numbers which count 2-independent vertex (or edge) subsets on the n-path. This approach is prompted by the fact that the square of the n-cycle is the starting point of constructions. Presented results complement, improve on, or extend the corresponding well-known Thomason's results.