[Xy-pic] Re: xypic question (braces)

[Ross Moore]

On Sun, 3 Aug 2003, Ken Goodearl wrote:

> Hi Ross,
>
> (Perhaps my question should have been sent to Kris Rose, but his email as
> given on the xypic home page doesn't function!)
>
> I have been trying to use xypic's frame extension to put braces, with a
> label indicating the number of terms, under a sequence of objects, as
> in the following rough sketch:
>
>      X   X   ...   X   X
>     \_________ _________/
>               V
>
>               n
>
> I expected the nib of the braces to be centered, but instead it occurs
> under one of the end objects; e.g.:
>
> $$\xymatrix{
> X \save[0,0];[0,4] **\frm{_\}} \restore &X &{\displaystyle\cdots}
> \save+<0ex,-4ex>\drop{n}\restore &X &X
> }$$

Try *merging* the matrix entries into a single object,
then center its reference-point:

$$\xymatrix{
 X \save[0,0].[0,4]!C *\frm{_\}} \restore &X &{\displaystyle\cdots}
 \save+<0ex,-4ex>\drop{n}\restore &X &X
 }$$


>
>
> $$\xymatrix{
> X \save[0,4];[0,0] **\frm{_\}} \restore &X &{\displaystyle\cdots}
> \save+<0ex,-4ex>\drop{n}\restore &X &X
> }$$
>
> According to paragraph 9.2 of the xypic reference guide, the nib will
> be aligned with the reference point of the object being braced, so I

Yes; that's why the !C was needed above.


> tried inserting the frame instructions at the center object:
>
> $$\xymatrix{
> X &X &{\displaystyle\cdots} \save[0,-2];[0,2] **\frm{_\}} \restore
> \save+<0ex,-4ex>\drop{n}\restore &X &X
> }$$
>
> However, this also ends up with the nib under the leftmost object.
>
>
> Is there some way to get the nib to be centered, or is that a feature
> not yet implemented?


> A secondary point: While it is easy enough to "drop" a label at an
> appropriate location near a brace, it might be nice to have a built-in
> command that would automatically align a label with the outer point of
> the nib of a brace.

Examine how the following code positions the label:

$$\xymatrix{
X \save[0,0].[0,4]!C *\frm{_\}}="mo" \restore &X &{\displaystyle\cdots}
\save"mo"+D,\drop++!U{n}\restore &X &X
}$$


Cheers

	Ross

>
>
> Thanks for your help.
>
> Ken Goodearl
>
>
>