Computing Complete Graph Isomorphisms and Hamiltonian Cycles from Partial Ones

We prove that computing a single pair of vertices that are mapped onto each other by an isomorphism $\varphi$ between two isomorphic graphs is as hard as computing $\varphi$ itself. This result optimally improves upon a result of G'{a}l et al. We establish a similar, albeit slightly weaker, result about computing complete Hamiltonian cycles of a graph from partial Hamiltonian cycles.