Now, the remaining 0.6% to resolve is spacing in the compiled output. With the cursor in front of the equation box I select Layout->Paragraph and change the justification from "justified" to "centered". However, the equation is still typeset across the entire line. The left-hand-side is left justified, the equals sign is centered and the right-hand-side is right justified on the line.
In AMS multline you can only change the indent of the first line with the length \multlinegap, see Herberts document.
If you are able to read german, you can have look at
http://wiki.lyx.org/pmwiki.php/LyX/LyXMathebefehle
where you find a LyX-Mathed documentation (Multiline-environments are described in Ch. 17, \cases in Ch. 3.5).
But better keep on using the \cases environment.
> I'm trying to enter the following equation: > > / > \mu_A(x) = < 1, if and only if x \in A > \ 0, if and only if x \notin A.
Here the exact command for this:
Ctrl-m \mu_A->(x)=\cases-> Ctrl-Enter
-> stands for the arrow button (press it directly after the last 's' for \cases)
Now fill out the boxes.
If you want more boxes, use the menu Edit->Math->AddColumn.
For text in formulas, use the math-panel and choose the last entry of its font menu (\textrm).
I hope this helps.
Here an example LyX-file:
------------------------------------------------------------------
#LyX 1.3 created this file. For more info see http://www.lyx.org/ \lyxformat 221 \textclass scrartcl \options fleqn \language ngerman \inputencoding auto \fontscheme ae \graphics default \float_placement h \paperfontsize 12 \spacing single \papersize a4paper \paperpackage widemarginsa4 \use_geometry 0 \use_amsmath 1 \use_natbib 0 \use_numerical_citations 0 \paperorientation portrait \secnumdepth 4 \tocdepth 4 \paragraph_separation skip \defskip medskip \quotes_language german \quotes_times 2 \papercolumns 1 \papersides 1 \paperpagestyle empty
\layout Standard
\begin_inset Formula \[ \mu_{a}(x)=\begin{cases} \\1 & \textrm{if and only if}\, x\in A\\ 0 & \textrm{if and only if}\, x\notin A\end{cases}\]
\end_inset
\the_end
