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I need to explain two different elements in two different lines of an equation in math mode. I'm using this piece of code:

h(z)=0 &\Leftrightarrow \sinh\frac{1}{z}=0 \markno{\Leftrightarrow}{According to ...} \frac{2}{z} = 2\pi i n  \\&\marknoup{\Leftrightarrow}{$n$ is whole} z = \frac{i}{\pi n}

(I'm shortening to make it clearer)

I'm using these macros:

\newcommand\markno[2]{\begin{array}[t]{@{\,}c@{\,}} #1 \\ \downarrow \\ \makebox[0pt]{\scriptsize #2} \end{array}}
\newcommand\marknoup[2]{\begin{array}[b]{@{\,}c@{\,}} \makebox[0pt]{\scriptsize #2}\\ \uparrow \\ #1 \end{array}}

The thing is, that LaTeX automatically spaces the lines, so that the "According two" is above (in height) "n is whole". I'm using two different arrows exactly to avoid that; I want them to overlap in height, in order to save space.

I tried using \vspace, but for some reason it doesn't work well for me.

share|improve this question
Write \\[-16pt] instead of \\ in the align* – egreg Jan 29 '12 at 22:35
That works. Thanks! – Hugo Jan 29 '12 at 22:36
up vote 3 down vote accepted

You can manually adjust the space between the lines in an align environment by adding an optional argument after the line end; e.g. \\[-5mm] instead of just \\. You could also try using \smash (see page 55 of the Not So Short Introduction, or sections 2 & 3 of A complement to \smash, \llap, and \rlap for details).

share|improve this answer
Thanks, it works. As for \smash, in the manual they use it in an inline equation. Can it be used in an align environment as well? – Hugo Jan 29 '12 at 22:43
Yes. I have added a reference to a document that describes \smash in more detail. – Ian Thompson Jan 29 '12 at 22:50

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