In math mode, the spaces between subscripts and letters are not always the same, but depending on the preceding letters!

See e.g. (different colours indicate different spaces)

Is it the feature of packages `{amssymb}`

`{amsmath}`

or ….

Equations using `{newtxmath}`

math font:

```
\begin{equation}
\left.
\begin{array} {c}
\displaystyle \frac{\partial\epsilon_f}{\partial t} + \nabla \cdot (\epsilon_f \boldsymbol{u}_f) = 0 \\
\displaystyle \frac{\partial (\rho_f \epsilon_f \boldsymbol{u}_f)}{\partial t} + \nabla \cdot (\rho_f \epsilon_f \boldsymbol{u}_f \boldsymbol{u}_f) = -\epsilon_f \nabla p + \mu \nabla^2(\epsilon_f \boldsymbol{u}_f) + \boldsymbol{f}_b
\end{array}
\right\}
\end{equation}
```

Is there any global setting/solution for this issue, i.e. to adjust the spacing automatically depending on the letters and subscripts?

**EDIT**: put `{bm}`

package after `amsmath`

and `font`

packages will significantly improve the spacing.

## Best Answer

After some testing I found that loading

`\usepackage{bm}`

significantly improves the spacing. AsDavidpoints out in the comment, the kerning is possible, but grouping plays a role here. Obviously his package`bm`

does better job on`\boldsymbol`

than`amsmath`

.Just remember to load`bm`

after`amsmath`

and after your font packages!