DEFINITIONG 函数 A function f whose domain and codomain are subsets of the set of real numbers is called strictly increasing if f(x)<f(y) whenever x <y and x and y are in the domain of f. Similarly, f is called strictly decreasing if f(x)>f(y) whenever x < y and x and y are in the domain of fF u n c t i o n s 函 数 DEFINITION 6. A function f whose domain and codomain are subsets of the set of real numbers is called strictly increasing if f(x) < f(y) whenever x < y and x and y are in the domain of f. Similarly, f is called strictly decreasing if f (x) > f (y) whenever x < y and x and y are in the domain of f