diff --git a/src/math/mathop.pysty b/src/math/mathop.pysty
index 26fdf45..6c33850 100644
--- a/src/math/mathop.pysty
+++ b/src/math/mathop.pysty
@@ -77,6 +77,7 @@ __END_OPTIONS_X__
 \DeclareSimpleMathOperator{sgn}
 \DeclareSimpleMathOperator{coker}
 \DeclareSimpleMathOperator{rank}
+\DeclareSimpleMathOperator{rk}
 \DeclareSimpleMathOperator{Mat}
 \DeclareSimpleMathOperator{ev}
 \DeclareSimpleMathOperator{tr}