Added ref to Perles

sagemath · Mar 20, 2017 · fdcb144 · fdcb144
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commit fdcb144
Showing 1 changed file with 2 additions and 1 deletion.
diff --git a/src/doc/en/thematic_tutorials/geometry/lectures.rst b/src/doc/en/thematic_tutorials/geometry/lectures.rst
@@ -606,7 +606,8 @@ The :code:`field` backend
 
 As it turns out, the rational numbers do not suffice to represent all combinatorial 
 types of polytopes. For example, Perles constructed a `8`-dimensional polytope with
-`12` vertices which does not have a realization with rational coordinates.
+`12` vertices which does not have a realization with rational coordinates, see
+Example 6.21 p. 172 of [Zie2007]_.
 Furthermore, if one wants a realization to have
 specific geometric property, such as symmetry, one also sometimes need
 irrational coordinates.