Update 3_numerical.rst

Author: hweifluids

source/3_numerical.rst

@@ -163,7 +163,7 @@ The continuous velocity field is reconstructed by trilinear interpolation of the
 .. math::
 
    \mathbf{u}=
-   \sum_{d_x,d_y,d_z\in\{0,1\}}
+   \sum_{d_i\in\{0,1\}}
    (1-d_x+(-1)^{d_x}\tau_x)
    (1-d_y+(-1)^{d_y}\tau_y)
    (1-d_z+(-1)^{d_z}\tau_z)