www.math.uwaterloo.ca/tsp This site provides a detailed history and many applications of the TSP.

en.wikipedia.org/wiki/Minimum_cost_spanning_tree This web page provides basic ideas about minimum-cost spanning trees, their applications, and extensions of this idea.

www-gap.dcs.st-and.ac.uk/~history/Biographies/Hamilton.html This site provides biographical information about William Rowan Hamilton, for whom Hamiltonian circuits are named.

www.ams.org/samplings/feature-column/fcarc-tsp www.ams.org/samplings/feature-column/fcarc-trees These sites provide some history and information about applications of the Traveling Salesman Problem and of minimum-cost spanning trees.