--- a/src/HOL/Groups.thy Fri Aug 12 17:49:02 2016 +0200
+++ b/src/HOL/Groups.thy Fri Aug 12 17:53:55 2016 +0200
@@ -602,7 +602,7 @@
\<^item> \<^emph>\<open>Partially Ordered Algebraic Systems\<close>, Pergamon Press, 1963
Most of the used notions can also be looked up in
- \<^item> @{url "http://www.mathworld.com"} by Eric Weisstein et. al.
+ \<^item> \<^url>\<open>http://www.mathworld.com\<close> by Eric Weisstein et. al.
\<^item> \<^emph>\<open>Algebra I\<close> by van der Waerden, Springer
\<close>