cone/faces.py: add the missing HilgertHofmannLawson reference that I cited.

diff --git a/mjo/cone/faces.py b/mjo/cone/faces.py
index 81b8421e94b28e0b9b9a68387a6b67f92c903a78..70706a8ca82383c25d98a17d28a333d8d942f83a 100644 (file)
--- a/mjo/cone/faces.py
+++ b/mjo/cone/faces.py
@@ -143,6 +143,10 @@ def dual_face(K,F):
 
     REFERENCES:
 
+    .. [HilgertHofmannLawson] Joachim Hilgert, Karl Heinrich Hofmann, and Jimmie
+       D. Lawson. Lie groups, convex cones and semigroups. Oxford Mathematical
+       Monographs. Clarendon Press, Oxford, 1989. ISBN 9780198535690.
+
     .. [Tam] Bit-Shun Tam. On the duality operator of a convex cone. Linear
        Algebra and its Applications, 64:33-56, 1985, doi:10.1016/0024-3795(85)
        90265-4.