I want to reproduce the shape of the following diagram :
And the closest thing I could manage with the xy package was :
$$
\xymatrix{
&B \ar[r] & H \ar[r] & B \ar[rd] \\
E'_{n+1} \ar[ru] \ar[r] & & E'_{n} \ar[r] & E_n \ar[r] & E_{n+1} }
$$
which gives
Is it possible to do so with this package?
Make a seven-column matrix:
\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}
\[
\xymatrix@C-1pc{
& B' \ar[rr]^{t_i} && H \ar[rr]^i && B \ar[dr]^{\tilde{v}} \\
E'_{n+1} \ar[ur]^{t_{\tilde{w}}} \ar[rr]_{t_{j_n}} &&
E'_n \ar[rr]_{\bar{u}} && E_n \ar[rr]_{j_n} && E_{n+1}
}
\]
\end{document}
The instruction @C-1pc reduces the space between columns.
With the missing arrows:
\documentclass{article}
\usepackage{amsmath}
\usepackage[all,cmtip]{xy}
\begin{document}
\[
\xymatrix@C-1pc{
& B' \ar[rr]^{t_i} && H \ar[rr]^i \ar[dr]^v && B \ar[dr]^{\tilde{v}} \\
E'_{n+1} \ar[ur]^{t_{\tilde{w}}} \ar[rr]_{t_{j_n}} &&
E'_n \ar[rr]_{\bar{u}} \ar[ur]^{t_w} && E_n \ar[rr]_{j_n} && E_{n+1}
}
\]
\end{document}
With tikz-cd:
\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz-cd}
\begin{document}
\[
\begin{tikzcd}[column sep=small]
& B' \arrow[rr,"t_i"] && H \arrow[rr,"i"] \arrow[dr,"v"] &&
B \arrow[dr,"\tilde{v}"] \\
E'_{n+1} \arrow[ur,"t_{\tilde{w}}"] \arrow[rr,"t_{j_n}"'] &&
E'_n \arrow[rr,"\bar{u}"'] \arrow[ur,"t_w"] && E_n \arrow[rr,"j_n"'] &&
E_{n+1}
\end{tikzcd}
\]
\end{document}
Labels are by default typeset on the left of the arrow (think to rivers), but with "<label>"' (note the apostrophe) the label goes on the other side. Alternatively, precede the label with the keyword swap.
\[...\]instead of$$...$$suggestion