Definition 17.1.5 (Intermediate Space).label Let $\catc$ be a subcategory of normed spaces over $K \in \RC$, $(E_{0}, E_{1}) \in \catc_{1}$ be a compatible couple in $\catc$, and $E \in \catc$, then $E$ is an intermediate space between $E_{0}$ and $E_{1}$ if there exists continuous inclusions