diff -r 371e814af6f0 -r f849fc1cb65e src/Doc/Logics/document/preface.tex
--- a/src/Doc/Logics/document/preface.tex	Wed Jul 18 12:21:55 2018 +0200
+++ b/src/Doc/Logics/document/preface.tex	Wed Jul 18 16:44:01 2018 +0200
@@ -56,7 +56,7 @@
 \begin{center}\small
   \begin{tabular}{l}
     \url{http://www.cl.cam.ac.uk/Research/HVG/Isabelle/library/} \\
-    \url{http://isabelle.in.tum.de/library/} \\
+    \url{https://isabelle.in.tum.de/library/} \\
   \end{tabular}
 \end{center}