In this paper, we study some complementary inequalities to Jensen's inequality for self-adjoint operators, unital positive linear mappings, and real valued twice differentiable functions. New improved complementary inequalities are presented by using an improvement of the Mond-Pečarić method. These results are applied to obtain some inequalities with quasi-arithmetic means.
Keywords: Mond-Pečarić method; converse of Jensen’s operator inequality; convex function; positive linear mapping; self-adjoint operator.