typesetting a matrix

Hi

in a .Rd file, in a \deqn{} environment, how do I typeset a matrix in 
latex?

I have

  \deqn{\left(\begin{bmatrix}a&b\\c&d\end{bmatrix}\right)}{omitted}

but this doesn't give me the 2-by-2 matrix desired.

Is there a manpage somewhere with a matrix that I could adapt?


--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743