I have the following triangle matrix with zeroes on the diagonal
\documentclass[a4paper]{memoir}
\usepackage{amsmath}
\begin{document}
\[
M = \begin{pmatrix}
0 & a_{1,2} & a_{1,3} & \dots & a_{1,n} \\
& 0 & a_{2,3} & \dots & a_{2,n} \\
& & 0 & \ddots & \vdots \\
& & & & 0
\end{pmatrix}
\]
\end{document}
The small problem I have now, is that the dots from \ddots point from a_{2,3} to the last zero. This could lead to the impression that the diagonal is not given by the zeroes. Anyone an idea?