Any suggestions for aligning a set of linear equations?

[jason-s]

What's the best way to align a set of linear equations in KaTeX? It doesn't support align* or eqnarray.

I can't seem to use \begin{aligned} to get the terms lined up nicely. The best I can do is this kludge:

\begin{matrix}X_\alpha \\ \\ X_\beta\end{matrix}
\begin{matrix}{}={} \\ \\{}={}\end{matrix}
\begin{matrix}\frac{2}{3}X_A \\ \\{}\end{matrix}
\begin{matrix}{}-{} \\ \\ {}\end{matrix}
\begin{matrix}\frac{1}{3}X_B \\ \\ \frac{1}{\sqrt{3}}X_B\end{matrix}
\begin{matrix}{}-{} \\ \\ {}-{}\end{matrix}
\begin{matrix}\frac{1}{3}X_C \\ \\{}\frac{1}{\sqrt{3}}X_C\end{matrix}

This SO question suggests using systeme but I'm guessing that's not supported in KaTeX.

[jason-s]

oh, looks like I can use array as in https://tex.stackexchange.com/a/328771/46076

\begin{array}{rcrcrc}
X_\alpha&=&\frac{2}{3}X_A&-&\frac{1}{3}X_B &-&\frac{1}{3}X_C \\
X_\beta&=& & &\frac{1}{\sqrt{3}}X_B &-&\frac{1}{\sqrt{3}}X_C \\
\end{array}

or

\begin{array}{rcrrr}
X_\alpha&=&\frac{2}{3}X_A&-\frac{1}{3}X_B &-\frac{1}{3}X_C \\
X_\beta&=&  &\frac{1}{\sqrt{3}}X_B &-\frac{1}{\sqrt{3}}X_C \\
\end{array}

but both have yucky-looking horizontal and vertical spacing.

[edemaine]

@jason-s If align* is what you want, KaTeX supports the nearly identical aligned environment. I don't know how to use that to align linear equations well though. What you're essentially asking for is \arraycolsep support, which is on the to-do list but not currently supported.

[jason-s]

I'm familiar with the use of aligned and it works very well for equations needing only one alignment point, as in

\begin{aligned}
X_\alpha&=\frac{2}{3}X_A-\frac{1}{3}X_B -\frac{1}{3}X_C \\
X_\beta&=\frac{1}{\sqrt{3}}X_B -\frac{1}{\sqrt{3}}X_C
\end{aligned}

but I would really like to align the terms so that the X_A, X_B, and X_C line up.

This seems like a common enough use case for systems of linear equations that KaTeX should support it somehow... just not sure how to get it the way you can in LaTeX. (not sure MathJax does either, however.)

[edemaine]

Like I said, what's missing is \arraycolsep support. Then your examples above will look great. And this is how I'd do it in LaTeX.

[jason-s]

Ah, ok, I'll wait patiently :-)

[jason-s]

Ooh! I just noticed alignedat in https://katex.org/docs/supported.html -- now to try it....

hmmm, this is a somewhat confusing feature:

\begin{alignedat}{4}
   X_\alpha &=\frac{2}{3}X_A &-& \frac{1}{3}X_B &- \frac{1}{3}&X_C \\
   X_\beta &= && \frac{1}{\sqrt{3}}X_B &- \frac{1}{\sqrt{3}}&X_C
\end{alignedat}

OK one more attempt with a fudge factor using manual spacing to help center the coefficients:

\begin{alignedat}{4}
   X_\alpha &=\tfrac{2}{3}X_A &-& \;\tfrac{1}{3}&X_B &- \enspace\tfrac{1}{3}&X_C \\
   X_\beta &= && \tfrac{1}{\sqrt{3}}&X_B &- \;\tfrac{1}{\sqrt{3}}&X_C
\end{alignedat}

ok minor tweaks:

\begin{alignedat}{4}
   X_\alpha &=\tfrac{2}{3}X_A &{}-{}& \;\tfrac{1}{3}&X_B &{}-{} \;\tfrac{1}{3}&X_C \\
   X_\beta &= && \tfrac{1}{\sqrt{3}}&X_B &- \tfrac{1}{\sqrt{3}}&X_C
\end{alignedat}

[edemaine]

alignat is how you'd enter math mode from vertical text mode, while alignedat is what you use when you're already in math mode. KaTeX only supports math mode, so it only supports the latter.

[jason-s]

ah -- ok that makes sense, thanks!

[ronkok]

Another explanation for alignat can be found at: https://www.onemathematicalcat.org/MathJaxDocumentation/TeXSyntax.htm#alignat

[snorky22]

It is all too bad that eqnarray is not supported because it is featured in the original LaTeX book by Leslie Lamport and used by quite a few people that I know. It is a classic.

[snorky22]

I have tried it and the alignedat environment is definitively not a substitute for the eqnarray environment
In eqnarray, the first part is flushed right, the second part is centered and the third part is flushed left.
You do not have that at all in alignedat.
Too bad, I will stick to Mathjax.