If the order formula_9 in the definition of monotonicity is replaced by the strict order formula_10, 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. A function may be called strictly monotone if it is either strictly increasing or strictly decreasing. Functions that are strictly monotone are one-to-one (because for formula_2 not equal to formula_3, either formula_13 or formula_14 and so, by monotonicity, either formula_15 or formula_16, thus formula_17.)