[Xy-pic] double arrows

[Ross Moore]

Hi Davide,

On 20/03/2014, at 12:18 AM, Davide Sangiorgi wrote:

> Dear Ross:
> Putting postive numbers does not seem to help:
> now the shortening of the line and the dots go at the other end of the lines.  Thanks.  If you happen to find out something, i'd be very interested (i use weak arrows a lot in my talks)

Here is a way to do it; but there is a catch.


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 \ar @<2pt>@{-}@*{[|(4)]}[d(.68)] 
 \ar @<-2pt>@{-}@*{[|(4)]}[d(.68)]
 \ar @2@{{}>}@*{[|(4)]}[d]

That number (.68) needs to be found by trial and error.
It is the fraction of the distance to the matrix position 
of the cell [d] from the target.
It will be different for different diagrams if you adjust the
row spacing, and different for longer arrows connecting cells
further apart, etc.

But if you use it a lot, some numbers with become quite common
for you.
In practice, you can pass it as an argument to a macro 
e.g. with argument pattern
    \dblar[#1](#2)
   ---> \ar @<2pt>@{-}@*{[|(4)]}[#1(#2)] etc.


> Best regards,
> DS


Hope this helps,

	Ross

------------------------------------------------------------------------
Ross Moore                                       ross.moore at mq.edu.au 
Mathematics Department                           office: E7A-206      
Macquarie University                             tel: +61 (0)2 9850 8955
Sydney, Australia  2109                          fax: +61 (0)2 9850 8114
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