Update the link to Diestel's book.

Kostero · Kostero · commit 3f9a3de2221a · 2024-10-23T02:39:29.000Z
diff --git a/src/geometry/planar.md b/src/geometry/planar.md
@@ -13,7 +13,7 @@ In this article we will deal with finding both inner and outer faces of a planar
 
 ## Some facts about planar graphs
 
-In this section we present several facts about planar graphs without proof. Readers who are interested in proofs should refer to [Graph Theory by R. Diestel](https://diestel-graph-theory.com/) (see [video lectures](https://www.youtube.com/@DiestelGraphTheory) based on this book) or some other book.
+In this section we present several facts about planar graphs without proof. Readers who are interested in proofs should refer to [Graph Theory by R. Diestel](https://www.math.uni-hamburg.de/home/diestel/books/graph.theory/preview/Ch4.pdf) (see also [video lectures on planarity](https://www.youtube.com/@DiestelGraphTheory) based on this book) or some other book.
 
 ### Euler's theorem
 Euler's theorem states that any correct embedding of a connected planar graph with $n$ vertices, $m$ edges and $f$ faces satisfies:
