a couple of questions on xy-pic

Question

I used xy-pic to typeset the following (long exact sequence)

\[ \xymatrix@1{ \cdots \ \ar[r] & \widetilde{H}_n(A) \ar[r]^{i_*} & \widetilde{H}_n(X)  \ar[r]^{j_*} & \widetilde{H}_n(X/A) \ar[r]^{\partial} & \widetilde{H}_{n-1}(A) \ar[r] & \ \cdots \ \ar[r] &  \widetilde{H}_0(X/A) \ar[r] & 0  } \] 

I was wondering if there is a (simple) way of equating each entry with something below it, in other words I would like a (short) vertical equals sign below each entry and something below that. I looked at the guide but I can't find where it explains how to do this.

Also, one more thing: is there a way to control the arrow length? I couldn't find this either...

Thank you very much for your help.

Answer 1

Treat it as a "real" matrix:

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

\[
\xymatrix@R=10pt@C-6pt{
  \cdots \ar[r] &
  \widetilde{H}_n(A) \ar[r]^{i_*} \ar@{=}[d] &
  \widetilde{H}_n(X)  \ar[r]^-{j_*} &
  \widetilde{H}_n(X/A) \ar[r]^-{\partial} \ar@{=}[d] &
  \widetilde{H}_{n-1}(A) \ar[r] &
  \cdots \ar[r] &
  \widetilde{H}_0(X/A) \ar[r] &
  0 \\
  & B & & C
}
\]

\end{document}

Notice the R=10pt to set the interrow spacing and C-6pt to reduce the intercolumn space. The labels over the arrows near the "big" objects are best announced with ^-{label} so that they'll be placed in the middle of the visible arrow, rather than midway from the cells' centers.