diff --git a/vdirsyncer/sync.py b/vdirsyncer/sync.py
index f58c375..f10587b 100644
--- a/vdirsyncer/sync.py
+++ b/vdirsyncer/sync.py
@@ -9,6 +9,9 @@
     type. This allows us not only to synchronize a local vdir with a CalDAV
     server, but also synchronize two CalDAV servers or two local vdirs.
 
+    The algorithm is based on the blogpost "How OfflineIMAP works" by Edward Z.
+    Yang. http://blog.ezyang.com/2012/08/how-offlineimap-works/
+
     :copyright: (c) 2014 Markus Unterwaditzer
     :license: MIT, see LICENSE for more details.
 '''