bmatrix: how to align elements' subscripts?
To align the 4 terms on their subscripts, I suggest you combine a bmatrix*[r]
environment (provided by the mathtools
package) with a \mathrlap{-}
instruction (also provided by the mathtools
package).
Update: to perform vertical alignment of the k
subscripts in the first row, I suggest you provide add a \vphantom{-}
superscript term to \mathbf{C}_k
term in the upper-right cell.
The result of the adjustments is shown in the lower matrix:
\documentclass{article}
\usepackage{mathtools} % for bmatrix* env. and \mathrlap macro
\begin{document}
\[
\begin{bmatrix}
\mathbf{P}^{-}_{k} & \mathbf{C}_k \\
\mathbf{C}_k & \mathbf{S}_k
\end{bmatrix}
\]
\[
\begin{bmatrix*}[r]
\mathbf{P}^{\mathrlap{-}}_{k} & \mathbf{C}^{\vphantom{-}}_k \\
\mathbf{C}_k & \mathbf{S}_k
\end{bmatrix*}
\]
\end{document}
Here are two possibilities:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\begin{bmatrix}
\mathbf{P}^{-}_{k} & \mathbf{C}^{\vphantom{-}}_k\\
\mathbf{C}_k & \mathbf{S}_k
\end{bmatrix}
\begin{bmatrix}
\mathbf{P}_{k}\llap{${}^{-}\mkern -3mu$} & \mathbf{C}_k\\
\mathbf{C}_k & \mathbf{S}_k
\end{bmatrix}
\]
\end{document}
Note: the second method uses manual adjustment for the horizontal position of the minus exponent (see the \mkern -3mu
).