diff --git a/doc/selection/thor_functors.rst b/doc/selection/thor_functors.rst
index 0980be4e1dbac33ea5d969a0e3d893185c58c072..a6c91ae4a8b36c6fd69d4064b92a0eb50e0262c8 100644
--- a/doc/selection/thor_functors.rst
+++ b/doc/selection/thor_functors.rst
@@ -472,3 +472,10 @@ the sum of the child momenta.
 .. _Gaudi: https://gaudi.web.cern.ch/gaudi/
 .. _MooreCache: https://gitlab.cern.ch/lhcb/Moore/-/tree/master/MooreCache
 .. _helper mixin: https://gitlab.cern.ch/lhcb/Rec/blob/e9a5d2feba21248f8691e1e8d1e160beeac2e9a4/Phys/FunctorCore/Functors/with_functors.h
+
+
+Mathematical operations on functors
+"""""""""""""""""""
+Mathematical functions are implemented in Functors.math - for example ::
+    import Functors.math as fmath
+    fmath.log(F.MINIPCHI2(pvs))