LaTeX.org

Thanks, I'm not used to \big \bigg and so on, now it works well ! (the error message is due to my quick shortening of my long document )
And thanks also for $$ to be \[ and \]... not used to it either !
Have a nice day

Well, I have the following (I simplified the inner matrices...) :

\documentclass[11pt,a4paper]{article}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{stmaryrd}
\usepackage{dsfont}
\usepackage{xfrac}
\usepackage{mathrsfs}
\begin{document ...

Hi everyone,

I need to write down large double square brackets and I'm struggling with it. Indeed, for the usual double square brackets I use \llbracket and \rrbracket , and for the usual large square brackets I use \left[ and \right] . But I can't manage to have large double square brackets ...