Alignment problem for a matrix

Question

I have written the following matrix on Latex:

However, I would like the different elements to be aligned vertically, i.e.:

I have been unable to accomplish this. My code is the following:

\[\phi_1(\mathbf{x}) = 
    \left(
    \begin{array}{l}
        x_1^2
        \\
        \sqrt{2}x_1x_2
        \\
        x_2^2
        \end{array}
   \right)\]

Do you know how could I fix this?

Answer 1

l columns are left-aligned, while c columns are center aligned, so just change l to c in the array specification. That said, I'd rather use pmatrix from amsmath here.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\phi_1(\mathbf{x}) = 
    \left(
    \begin{array}{c}
        x_1^2
        \\
        \sqrt{2}x_1x_2
        \\
        x_2^2
        \end{array}
   \right)
\]
With \texttt{pmatrix}:
\[
\phi_1(\mathbf{x}) = 
    \begin{pmatrix}
        x_1^2
        \\
        \sqrt{2}x_1x_2
        \\
        x_2^2
        \end{pmatrix}
\]
\end{document}