1. American Mathematical Society Colloquium Publications, Vol. 25;Birkhoff, Garrett,1948
2. Two brief formulations of Boolean algebra;Byrne, Lee;Bull. Amer. Math. Soc.,1946
3. Trees, lattices, order, and betweenness;Sholander, Marlow;Proc. Amer. Math. Soc.,1952
4. Medians and betweenness;Sholander, Marlow;Proc. Amer. Math. Soc.,1954