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On the Complexity of Determining Whether there is a Unique Hamiltonian Cycle or Path

The decision problems of the existence of a Hamiltonian cycle or of a Hamiltonian path in a given graph, and of the existence of a truth assignment satisfying a given Boolean formula $C$, are well-known {\it NP}-complete problems. Here we study the problems of the {\it uniqueness} of a Hamiltonian cycle or path in an undirected, directed or oriented graph, and show that they have the same complexity, up to polynomials, as the problem U-SAT of the uniqueness of an assignment satisfying $C$. As a consequence, these Hamiltonian problems are {\it NP}-hard and belong to the class~{\it DP}, like U-SAT.