The functions that use different formulas on different parts of their domains are called "piecewise-defined functions".
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Определение piecewise function в Английский Язык Английский Язык словарь
In mathematics, a piecewise-defined function f(x) of a real variable x is a function whose definition is given differently on disjoint subsets of its domain
In mathematics, a piecewise-defined function f(x) of a real variable x is a function whose definition is given differently on disjoint subsets of its domain