diff --git a/Mathematics/5th/Introduction_to_nonlinear_elliptic_PDEs/Introduction_to_nonlinear_elliptic_PDEs.tex b/Mathematics/5th/Introduction_to_nonlinear_elliptic_PDEs/Introduction_to_nonlinear_elliptic_PDEs.tex
index 2af0c38..e5e949a 100644
--- a/Mathematics/5th/Introduction_to_nonlinear_elliptic_PDEs/Introduction_to_nonlinear_elliptic_PDEs.tex
+++ b/Mathematics/5th/Introduction_to_nonlinear_elliptic_PDEs/Introduction_to_nonlinear_elliptic_PDEs.tex
@@ -258,7 +258,7 @@
   \begin{definition}
     Let $H$ be a $\KK$-Hilbert space and $K:H\to H$ be a compact operator. We define the \emph{resolvent set} of $K$ as:
     $$
-      \rho(K)=\{\lambda\in \KK: \lambda-K \text{ is invertible}\}
+      \rho(K)=\{\lambda\in \KK: \lambda\id-K \text{ is invertible}\}
     $$
     and the \emph{spectrum} of $K$ as:
     $$