Amsmath: boxed and split equation

[Ajit J. Thakkar]

Using the following construct
\begin{equation}
\boxed{ \begin{split} a &= b\\ & = c \end{split} }
\end{equation}
produces a two-line equation with a box around it as desired.

However amsmath v2.08 issues the following message:
Package amsmath Warning: Cannot use `split' here;
(amsmath) trying to recover with `aligned' on input line
235.

What is the correct method for doing this with amsmath?

[Jonathan LF King]

Hello Ajit J. Thakkar

I suggest
\begin{equation}
\boxed{ \begin{aligned} a &= b\\ & = c \end{aligned} }
\end{equation}

We don't have the new amsmath here; I did however test it successfully under
the former amsmath pkg. Best Wishes, -Jonathan
--
Prof. Jonathan LF King Mathematics dept, Univ. of Florida
<squ...@math.ufl.edu>, <http://www.math.ufl.edu/~squash/>

[Michael J Downes]

"Ajit J. Thakkar" <aj...@unb.ca> writes:

You should be able to just use \begin{aligned}...\end{aligned} instead
of \begin{split}...\end{split}.

The reason for the warning is that the amsmath equation environments are
classified into two main types: full-display and subobject. Full-display
objects take up the entire "current line" and do not allow anything else
on either side (except possibly an equation number). Examples of
full-display environments are equation, align, and gather. Examples of
subobject are aligned, gathered, cases, etc.

The "split" environment is something of a special case that does not
fall cleanly into either type, but historically it was a full-display
object and was not supposed to have anything on either side of it. When
"split" was ported from AMS-TeX to LaTeX this distinction became blurred
and in some cases it was possible to put extra material in there on the
left or right of a "split" object even though it was never intentionally
designed to support that. As of amsmath version 2.0 a warning was added,
as you have discovered. But it really should have been flagged as an
error in previous versions.

Because adding a box around an object involves putting extra material on
either side, it is supported mainly for subobjects, not full-display
objects. If you want to box a full-width object things can get rather
complicated. There is an item in the amsmath FAQ, however, that
addresses the question.

Regards, Michael Downes
tech-s...@ams.org