Definition 17.1.8 (Interpolation Exponent).label Let $\catc$ be a subcategory of normed spaces over $K \in \RC$, $\catc_{1}$ be its categories of compatible couples, $F: \catc_{1} \to \catc$ be an interpolation functor, and $\theta \in [0, 1]$, then $F$ is of exponent $\theta$ if there exists $C \ge 0$ such that for every $(E_{0}, E_{1}), (F_{0}, F_{1}) \in \catc_{1}$ and $T \in \text{Mor}_{\catc_1}((E_{0}, E_{1}); (F_{0}, F_{1}))$

If $C = 1$, then $F$ is of exact exponent $\theta$.