Curved arrow in xy-pic

Question

\documentclass{article}
\usepackage{graphicx}
\usepackage[all]{xy}
\begin{document}
$\xymatrix{& A \ar[d] & B \ar[d] & C \ar[d] &  \\
D \ar[r] & E \ar[d] \ar[r] & F  \ar[r] \ar[d] & G \ar[d] \ar`r[d]`[l]`[llld]`[dddll][dddll] & \\
H \ar[r] & I \ar[d] \ar[r] & J \ar[r] \ar[d] & K \ar[r] \ar[d] & L \\
M \ar[r] & N \ar[d] \ar[r] & O \ar[d] \ar[r] & P \ar[r] \ar[d] & Q\\ & R & S & T &}$
\end{document}

The curved arrow:

{ar`r[d]`[l]`[llld]`[dddll][dddll]}

I'm using xy-pic to draw a commutative diagram with a curved arrow.

The code above gives the result in the picture, but I want a curved arrow like the blue one. I tried to adjust the code in the curved arrow but failed.

My question:

How can I draw the desired arrow using xy-pic?

An alternative method would be like this, but it will enlarge the whole diagram. And the relative coordinate is not valid for fractions.

Thanks for your time and effort!

Answer 1

You need to double the number of columns, reducing the spacing between columns.

\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}

\begin{document}

\begin{equation*}
\xymatrix@C=1pc{
&& A \ar[d] && B \ar[d] && C \ar[d] &&  \\
D \ar[rr] && E \ar[d] \ar[rr] && F \ar[d] \ar[rr] && G \ar[d]
  \ar`r[d]`[ll]`[llllld]`[dddllll][dddllll]
&& \\
H \ar[rr] && I \ar[d] \ar[rr] && J \ar[d] \ar[rr] && K \ar[d] \ar[rr] && L \\
M \ar[rr] && N \ar[d] \ar[rr] && O \ar[d] \ar[rr] && P \ar[d] \ar[rr] && Q\\
&& R && S && T &&
}
\end{equation*}

\end{document}