diff --git a/org/qmckl_ao.org b/org/qmckl_ao.org
index 6eff600..2a97574 100644
--- a/org/qmckl_ao.org
+++ b/org/qmckl_ao.org
@@ -4402,8 +4402,8 @@ assert(0 == test_qmckl_ao_power(context));
               \frac{\partial }{\partial z^2} \right) P_l
               \left(\mathbf{r},\mathbf{R}_i \right) &  = &
             a(a-1) (x-X_i)^{a-2} (y-Y_i)^b (z-Z_i)^c + \\
-         && b(b-1) (x-X_i)^a (y-Y_i)^{b-1} (z-Z_i)^c + \\
-         && c(c-1) (x-X_i)^a (y-Y_i)^b (z-Z_i)^{c-1}.
+         && b(b-1) (x-X_i)^a (y-Y_i)^{b-2} (z-Z_i)^c + \\
+         && c(c-1) (x-X_i)^a (y-Y_i)^b (z-Z_i)^{c-2}.
    \end{eqnarray*}
 
    ~qmckl_ao_polynomial_vgl~ computes the values, gradients and