[Xy-pic] shorter arrows

[Ross Moore]

Hello Jonas,

On 26/08/2010, at 10:09 PM, Jonas Frey wrote:

> Hi,
> 
> I'm looking for a way to draw arrows in xymatrix which stretch not over the whole distance between to nodes, but only over  a certain percentage. I want to use this to draw 2-cells in commutative squares.

Xy-pic has a 2-cell feature, which lets you position
a short double-arrow within a diagram, indicating
the 2-cell nature of that portion of it.
It was developed for, and used extensively in,
Ross Street's book on "Quantum Groups".

Does this not do the kind of thing that you want?
Or do you want longer arrows, that adapt to the size
of the square/rectangle or other polygonal area defined
by the composition of arrows in the 2-cell?
that is an awful lot harder, as you'll need to work out
a way of specifying where to start and end the arrow.

> 
> Until now, I have done this by drawing invisible frames around source and target nodes, as in the following example:
> \[
> \xymatrix{
> FA
>     \ar[r]^{Ff}
>     \ar[d]_{\eta_A} &
> FB
>     \ar[d]^{\eta_B}
> \\
> GA
>     \ar[r]_{Gf} &
> GB
> \\
> %
> \ar@{<=}"1,2"*+++\frm{};"2,1"*++\frm{}^{\eta_f}
> }
> \]
> However, this seems overly complicated and has the disatvantage that the invisible frames reach in all directions, and so make the entire diagram bigger, resulting in too much space above and beneath the diagram in the document.
> 
> Is there a easier/better way to do this?
> 
> Thanks in advance,
> 
> Jonas


Hope this helps,

	Ross

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Ross Moore                                       ross.moore at mq.edu.au 
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