started working on eigenvalue

chapters/sections/eigenvalue.tex

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+\documentclass[../../script.tex]{subfiles}
+% !TEX root = ../../script.tex
+
+\begin{document}
+\section{Eigenvalue problems}
+
+\begin{defi}
+    Let $A \in \field^{n \times n}$. Then $\lambda \in \field$ is called an eigenvalue of $A$, if
+    \[
+        \exists v \in \field^n, ~v \ne 0: ~~Av = \lambda v
+    \]
+    Such a vector $v$ is called eigenvector. We call 
+    \[
+        \set[Av = \lambda v]{v \in \field^n} =: E_{\lambda}
+    \]
+    eigenspace belonging to $\lambda$
+\end{defi}
+\end{document}