Takayama, Akira (1994), Analytical Methods in Economics, University of Michigan Press, стр. 54, ISBN9780472081356, „An often seen confusion is a "concave set". Concave and convex functions designate certain classes of functions, not of sets, whereas a convex set designates a certain class of sets, and not a class of functions. A "concave set" confuses sets with functions.”
