In mathematics, a monotonic function (or monotone function) is a function which preserves the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order theory.
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Monotonicity in calculus and analysis
In calculus, a function f defined on a subset of the real numbers with real values is called monotonic (also monotonically increasing, increasing or nondecreasing), if for all x and y such that x ≤ y one has f(x) ≤ f(y), so f preserves the order (see Figure 1). Likewise, a function is called monotonically decreasing (also decreasing, or nonincreasing) if, whenever x ≤ y, then f(x) ≥ f(y), so it reverses the order (see Figure 2).
If the order ≤ in the definition of monotonicity is replaced by the strict order <, then one obtains a stronger requirement. A function with this property is called strictly increasing. Again, by inverting the order symbol, one finds a corresponding concept called strictly decreasing. Functions that are strictly increasing or decreasing are onetoone (because for x not equal to y, either x < y or x > y and so, by monotonicity, either f(x) < f(y) or f(x) > f(y), thus f(x) is not equal to f(y)).
The terms "nondecreasing" and "nonincreasing" are meant to avoid confusion with "strictly increasing" respectively "strictly decreasing", but should not be confused with the (much weaker) negative qualifications "not decreasing" and "not increasing"; see also strict. When functions between discrete sets are considered in combinatorics, it is not always obvious that "increasing" and "decreasing" are taken to include the possibility of repeating the same value at successive arguments, so one finds the terms weakly increasing and weakly decreasing to stress this possibility.
The term monotonic transformation can also possibly cause some confusion because it refers to a transformation by a strictly increasing function. Notably, this is the case in Economics with respect to the ordinal properties of a utility function being preserved across a monotonic transform (see also monotone preferences).^{[1]}
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