 The continuous functions from \(D\) to \(E\), for cpo’s \({\langle D,\sqsubseteq _D \rangle}\) e \({\langle E,\sqsubseteq _E \rangle}\), form a cpo \([D \to E]\), ordered pointwise by setting, for \(f,g:D \to E\):   \[f \sqsubseteq g \Longleftrightarrow \forall d \in D. f(d) \sqsubseteq _E g(d).\]   \([D \to E]\) is a domain if \(D\) and \(E\) are, and \({\texttt{fix}(\cdot)}:[D \to D] \to D\) is continuous. A further construction on cpo’s which also extends to domains and is very frequently used is cartesian product: given cpo’s \(D,E\), their cartesian product is defined as the set \(D \times E\) of pairs \({\langle d,e \rangle}\) where \(d \in D\) and \(e \in E\), ordered pointwise: \({\langle d,e \rangle} \sqsubseteq {\langle d',e' \rangle}\) if and only if \(d \sqsubseteq _D d'\) and \(e \sqsubseteq _E e'\). We can summarize these constructions in categorical language (Plotkin 1978, Other Internet Resources), saying that the category whose objects are domains and whose morphisms are the continuous functions is cartesian closed.