## Triangular system of linear equations in LaTeX

Today, I wanted to write a system of linear equations in a triangular form using $$\LaTeX$$. As you may know, $$\LaTeX$$ and especially the amsmath package offers many ways to align equations. I wanted to achieve something like

\begin{alignat*}{2}% Use alignat when you wish to have individual equation numbering &l_{00} y_{0} && = b_{0} \\ &l_{10} y_{0} + l_{11} y_{1} && = b_{1} \\ &l_{20} y_{0} + l_{21} y_{1} + l_{22} y_{2} && = b_{2} \end{alignat*}

I tried several ways including the align or equation (with split) environment. But for some reason, all of them messed up.

Finally, I found the alignat environment, also from the amsmath package, which did what I wanted. This environment lets you explicitly set the spacing between the equations. The MWE looks like


\documentclass{scrartcl}
\usepackage{amsmath}

\begin{document}

\begin{alignat*}{2}% Use alignat when you wish to have individual equation numbering
&l_{00} y_{0}                               && = b_{0} \\
&l_{10} y_{0} + l_{11} y_{1}                && = b_{1} \\
&l_{20} y_{0} + l_{21} y_{1} + l_{22} y_{2} && = b_{2}
\end{alignat*}

\end{document}



As the official documentation (p. 7) explains, the mandatory argument is the number of “equation columns” which can be calculated as the maximum number of & in a row + 1 divided by 21.

1. Don't ask me why we have to present an argument which could easily be calculated by someone else – a computer machine for instance ;-). But, of course, that little inconvenience is easily accepted considering the beautiful output we get as a reward.