LaTeX matrix when publishing

Just had the very same issue! Strangely enough the

pmatrix

Call works just fine. If I use the following:

%%
% 
% $$\hat{\mathbf{x}}_k = \pmatrix{x \cr
%                                 y \cr
%                                 \dot{x} \cr
%                                 \dot{y} \cr}$$

then I get a nicely formatted vector, but with parentheses. If I use

'bmatrix' 

I get the same error you report. In the end I decided that I'd just have to live with parentheses rather than try and use some extreme TeX-Fu. Hope this helps.