Unconstrained optimization problem:
$$ \operatorname*{minimize}_{x \in \R^d} \; f(x) $$
Constrained optimization problem:
$$ \begin{aligned} \operatorname*{minimize}_{x \in \R^d} \;\; & f(x) \\ \text{subject to } \;\;& c_i(x) \leq 0, \; i = 1, \ldots, m \\ & h_j(x) = 0, \; j = 1, \ldots, n \end{aligned} $$
Can be written more abstractly as:
$$ \operatorname*{minimize}_{x \in S} \; f(x) $$
$S \subseteq R^d$ is the optimization domain, e.g.:
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Optimization is used as a primary computational tool in many fields: