Some methods for constructing new operator monotone functions from old ones

Abstract

We observe that if f is a continuous function on an interval I and x_0 \in I, then f is operator monotone if and only if the function (f(x) - f(x_0)/(x - x_0) is strongly operator convex. Then starting with an operator monotone function f_0, we construct a strongly operator convex function f_1, an (ordinary) operator convex function f_2, and then a new operator monotone function f_3. The process can be continued to obtain an infinite sequence which cycles between the three classes of functions. We also describe two other constructions, similar in spirit. We prove two lemmas which enable a treatment of those aspects of strong operator convexity needed for this paper which is more elementary than previous treatments. And we discuss the functions phi such that the composite phi \circ f is operator convex or strongly operator convex whenever f is strongly operator convex.