| − | An '''operator''' is a map from function space to function space. If the range of an operator is on the [[real line]] or in the [[complex plane]], the operator is usually called a [[functional]] instead. | + | An '''operator''' is a map from function space to function space. If the range of an operator is on the [[real line]] or in the [[complex plane]], the operator is usually called a [[functional operator|functional]] instead. |