DOC Fix typo: R_ij -> R_{ij} for Davies-Bouldin Index
@@ -1809,7 +1809,7 @@ this index, similarity is defined as a measure :math:`R_{ij}` that trades off:
the centroid of that cluster -- also know as cluster diameter.
- :math:`d_{ij}`, the distance between cluster centroids :math:`i` and :math:`j`.
-A simple choice to construct :math:`R_ij` so that it is nonnegative and
+A simple choice to construct :math:`R_{ij}` so that it is nonnegative and
symmetric is:
.. math::
