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I'm trying to make a diagram with arrows returning to the same entry using xymatrix. The following example works perfectly fine.

V \ar@(dl,ul)[]^{h'} \ar@<0.5ex>[r]^{i} & W \ar@(dr,ur)[]_{h} \ar@<0.5ex>[l]^{p} 

However, if I try to change V to (V,d_V), I only get error messages. I tried already to place it into {}, but it makes no difference. Typing

{(V,d_V)} \ar@(dl,ul)[]^{h'} \ar@<0.5ex>[r]^{i} & {(W,d_W)} \ar@(dr,ur)[]_{h} \ar@<0.5ex>[l]^{p} 

I still get error messages like

! Undefined control sequence.
\splineendScan@ ...s@ \expandafter {\splineedges@ 
                                              }\xylowtolerance@ \splined...

I hope someone can help me.

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This may be a defect of xy-pic, you can change V to (x,y) but not (X,Y), and for (x,y) the result is unsatisfactory. – Ma Ming May 23 '11 at 17:14
up vote 1 down vote accepted

Xy-pic sometimes has problems with curving or turning arrows when the nodes are too large. (I've never been able to predict when the problems would arise.) For a crude workaround, you can use

  *[r]{\;(V,d_V)\;} \ar@<0.5ex>[rr]^{i} \ar@(dl,ul)[]^{h'} 
  && *[l]{\;(W,d_W)\,} \ar@<0.5ex>[ll]^{p} \ar@(dr,ur)[]_{h} 

We used *[r]{node} to shift that first node to the right and *[l]{node} to shift that second node to the left. We also padded each node with a \; at the beginning and end, and we made the second node two jumps to the right, and made the arrows into \ar[rr] and \ar[ll], to leave room for the arrows (which were otherwise crowded by the shifted nodes). It's crude, but it mostly works: enter image description here

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Thank you, it worked perfectly! – pyro4erebus May 23 '11 at 21:35

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