Document Classes ⇒ multiple alignment in an equation

[daviddoria]

using amsmath \begin{align}, I would like to do something like this

a + 2b + 3c = 4
3a + + 7c = 5

where if there are missing terms, I can use another align character to line up the + signs, as well as the equal signs

I tried this:

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\begin{align*}
x_1 &+ 4x_2 &= -2 \\
x_1 &+ 2x_2 &= 6 \\
2x_1 &+ 3x_2 &= 1 
\end{align*}

But it kind of separates the equations into two columns. I didn't see an explanation of how to do this the amsldoc

can someone give me some pointers?

Thanks,
Dave

[Stefan Kottwitz]

Hi Dave,

use the alignat* environment.

Stefan

[daviddoria]

The doc says you should use (max number of & in a line + 1)/2 as the argument. I tried this:

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\begin{alignat*}{4}
x &+ y &+ z &= -2 \\
x &+ y &+ z &= 6 \\
x &+ &+ z &= 1 
\end{alignat*}

and

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\begin{alignat*}{2}
x &+ y &+ z &= -2 \\
x &+ y &+ z &= 6 \\
x &+ &+ z &= 1 
\end{alignat*}

and they seem to produce the same result. What is that argument saying exactly?

Dave

[Stefan Kottwitz]

Hi Dave,

the argument specifies the number of columns. It should not really make a difference if you choose the number too high, but it would not compile with a value that's too low. For your example 2 columns are necessary, but you could declare more, like you've tested with 4.

Stefan

[Juanjo]

Consider the mathematical expressions as divided in columns. As Stephan_K says, the argument of alignat is the number of columns. Inside each column fix an aligment point. Then, in every line, mark with & each aligment point and also the end of each column (except the last one). So, if you have n columns, you should have 2n-1 & signs.

For the example of your first post, you could try

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\begin{alignat*}{2}
x_1 &+{}& 4x_2 &= -2 \\
x_1 &+{}& 2x_2 &= 6 \\
2x_1&+{}& 3x_2 &= 1 
\end{alignat*}

Note the pairs of braces. They are generally needed if a binary operation falls before of after a column separation to get proper spacing. To see this, remove the braces and compare. I add a more complex example:

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\begin{alignat*}{4}
-7x_1 & & &+{}& 41x_3 &-{}& 14x_4&=9, \\
23x_1 &-{}& x_2 &+{}& 9x_3 & & &=-22, \\
x_1 &+{}& 10x_2 & & &+{}& 291x_4&=53, \\
& & 3x_2 &+{}& x_3 &+{}& 21x_4&=14.
\end{alignat*}

To obtain the same expressions, there are usually some possible alternatives on where putting the aligment points and columns. This may require some experimentation.

[daviddoria]

excellent, thanks everyone!