Monotonic conditions. Extremum of function
Theorem 1. Let be continuous on [a, b] and differentiable on (a, b). is a constant function if and only if for all . Definition 1. A function is said to be monotonic increasing (resp. monotonic decreasing) or simply increasing (resp. decreasing) on an interval if and only if , if then (resp. , if , then ). Definition 2. A function is said to be strictly increasing (resp. strictly decreasing) on an interval if and only if , if then (resp. , if , then ). Theorem 2. Let be continuous on [a, b] and differentiable on (a, b). Then (a) if is strictlyincreasing on [a, b]; and (b)if is strictly decreasing on [a, b]. Definition 3. A neighborhood of a point is an open interval containing , i.e. is a neighborhood of for some . Definition 4. A function is said to attain a relative maximum (minimum) at a point if () in a certain neighborhood of , i.e. such that () for .
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