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I'm trying to enter a long chain of inequalities in math display mode. If I enter the expressions as I want them to, LyX will continue displaying them off the screen - so that they don't show up when I compile the pdf (and I cannot see what I am typing). Is there a way I can enter a long chain of expressions in display mode so that LyX displays them nicely? My only option so far has been to exit display mode when I'm too far right of the screen, hit enter and go back into display mode to enter the next line. However, this doesn't look too great on the pdf, and I have no idea when is a good time to shift to the next line anyway. I'm assuming there is a proper way to do this.

The Substitute
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    Welcome to TeX.SE. Besides the breqn package there probably isn't an easy solution to automatic line breaking for display math. – Peter Grill Nov 19 '12 at 05:41
  • @PeterGrill; silly question: is "breqn package" a way I'm supposed to enter the expressions or a software package that's compatible with LyX? – The Substitute Nov 19 '12 at 05:44
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    No its a LaTeX package that you include in your preamble with \usepackage{breqn}, and then use the dmath environment. But I believe is has issues and may be better to just break the equations manually, after finding where they need to break. Have a look at the breqn questions. – Peter Grill Nov 19 '12 at 05:50
  • Certainly breqn is not a magic bullet - breaking equations is hard work, and you still have to intervene manually in some cases. Also, I doubt that Lyx 'knows' about breqn, so you will have to use direct LaTeX entry ('ERT') to use it. – Joseph Wright Nov 19 '12 at 08:23
  • The standard LaTeX way is to use an eqnarray environment and manually places the break points. The improved way is to use the alignment environments provided by the amsmath package; again you manually specify the break points; the spacing is better than eqnarray. Finally, as others mention, there is the breqn package that will automatically break long equations; tweaking its parameters to get output one likes can be frustrating. – Andrew Swann Nov 19 '12 at 09:30
  • @AndrewSwann: eqnarray is not recommended. See \eqnarray vs \align. – Peter Grill Nov 19 '12 at 10:18
  • @PeterGrill That is what I tried to say. But with the caveat that \eqnarray is built in to the latex core, \align needs the amsmath package. – Andrew Swann Nov 19 '12 at 10:21

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As people mentioned in the comments you should use eqnarray or align By default you can do it by pressing Ctrl-Enter in math mode, this will create an eqnarray environment. By pressing Ctrl-Enter again, you can get additional lines. Spacing should be much better than acieved by separate displayed math environments.

To use the AMS align environment, you should tick "Use AMS math package" (first untick "Use AMS math package automatically") in Document->Settings->Math Options. This is probably the best thing to do, as align is much better looking than eqnarrary.

Guy
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    align is not only better looking than eqnarray; the latter is seriously flawed under many respects and should never be used. – egreg Feb 01 '13 at 09:50