$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$

Corollary Let $f: X \rightarrow Y$ be a Kan fibration of simplicial sets. Then the induced map $\operatorname{Ex}(f): \operatorname{Ex}(X) \rightarrow \operatorname{Ex}(Y)$ is also a Kan fibration.

Proof. We must show that every lifting problem

\[ \xymatrix@C =50pt@R=50pt{ \Lambda ^{n}_{i} \ar [r] \ar [d] & \operatorname{Ex}(X) \ar [d]^{ \operatorname{Ex}(f) } \\ \Delta ^{n} \ar [r] \ar@ {-->}[ur] & \operatorname{Ex}(Y) } \]

admits a solution. This follows by applying Remark to the associated lifting problem

\[ \xymatrix@C =50pt@R=50pt{ \operatorname{Sd}(\Lambda ^{n}_{i}) \ar [r] \ar [d] & X \ar [d]^{ f} \\ \operatorname{Sd}(\Delta ^{n}) \ar [r] \ar@ {-->}[ur] & Y, } \]

since the left vertical map is anodyne by virtue of Proposition $\square$