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I need to use the xy maths package (because I need to make it into a PNG file for Wordpress and mathurl.com accepts the xy template) to make a commutative diagram in which there are diagonal 4 shorter arrows inside the diagram, directed inward. The best I can do is

\usepackage[all]{xy}
\xymatrix{Y' \ar[r]^{g'}
\ar[d]_{f'} & X' \ar[d]^{f} \\
Y \ar[r]_{g} & X 
\\
%
\ar@{=>}"1,2"*++++\frm{};"2,1"*++\frm{}}

So I can only get one arrow, which needs to be higher up to the right.

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Welcome to TeX.sx! You can mark code by selecting it and clicking the button marked {} above the text field (or hitting Ctrl + K). –  Torbjørn T. Mar 31 '12 at 13:58
    
Thanks Torbjørn! –  j4fj49 Mar 31 '12 at 14:10
    
Maybe I don't fully understand the question; is something like this what you need? \xymatrix{Y' \ar[r]^{g'} \ar[d]_{f'} & X' \ar[d]^{f} \Y \ar[r]_{g} & X \\ar@{=>}"1,2"*++\frm{};"2,1"*++\frm{} \ar@{=>}"1,1"*++\frm{};"2,2"*++\frm{}} –  Gonzalo Medina Mar 31 '12 at 14:49
    
Almost, except the arrows should be shorter with tips at the centre (so those are coming down from the top two corners, and there should also be 2 more arrows coming up from below. –  j4fj49 Mar 31 '12 at 14:51
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1 Answer

up vote 4 down vote accepted

I'd use additional rows and columns:

\xymatrix@R.8pc@C.8pc{
Y' \ar[rr]^{g'} \ar[dd]_{f'} && X' \ar[dd]^{f} \\
 & \mbox{\phantom{$x$}} \ar@{<=}[ul] \ar@{<=}[ur] \ar@{<=}[dl] \ar@{<=}[dr]   & \\
Y \ar[rr]_{g} && X
}

enter image description here

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Thanks egreg!!! –  j4fj49 Apr 2 '12 at 11:11
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