Kruskal, 1956, On the Shortest Spanning Subtree of a Graph and the Traveling Salesman Problem, Proc. Amer. Math. Soc., 7, 48, 10.1090/S0002-9939-1956-0078686-7
Borůvka, 1926, On a Minimal Problem, Práce Moravské Pridovedecké Spdecnosti, 3
Courant, 1941, What is Mathematics, 374