Double square brackets

Question

In many equation editors (e.g. MS Word 2007 and Daum Equation Editor), there's a double square bracket. Is there a TeX equivalent for [[, ((, )), and ]]?

Answer 1

If you only want then occasionally, perhaps you can just roll-your-own by using a little bit of negative space, like this

Maths in double brackets $[\![ e=mc^2 ]\!]$ or double parentheses $(\!( a^2 )\!)$. 

Note that they will not scale nicely with \left and \right because of the fixed negative space.

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If you'll be using them a lot, try using a package like stmaryrd that defines commands for these symbols, as recommended in the comments.

\documentclass{article}
\usepackage{stmaryrd}

\begin{document}

\begin{equation}
  \llbracket     1 \rrbracket       \quad
  \llparenthesis 2 \rrparenthesis   \quad
  \llceil        3 \rrceil          \quad
  \llfloor       4 \rrfloor         \quad
\end{equation}

\end{document}

Note especially the fancier display for double parentheses.

As a side note, DeTeXify is getting better and better at recognizing these symbols, so you can check there for more package-based options.

Answer 2

These commands are \lBrack/\rBrack, and either \lParen and \rParen or \llparenthesis and \rrparenthesis in unicode-math or several 8-bit packages, including stix, stix2 and fdsymbol. In Unicode, you can also use the symbols ⟦, ⦅, ⦇, etc.

You might find the \DeclarePairedDelimiter command from mathtools very convenient, especially if you need to specify a size.

\documentclass{article}
\usepackage{mathtools}
\usepackage{newcomputermodern} % Loads unicode-math

\DeclarePairedDelimiter\Brackets{\lBrack}{\rBrack}
\DeclarePairedDelimiter\Parens{\lParen}{\rParen}

\begin{document}
\begin{gather*}
   \lBrack x \rBrack \quad \lParen y \rParen \quad
   \llparenthesis z \rrparenthesis \\
   \Brackets{u} \quad \Parens{v}
\end{gather*}
\end{document}

Answer 3

In the case that you would have a conflict between stmaryrd and another package, or that you dislike some redefinitions of symbols (such as \bigtriangledown), you may want to use the following line to import only the commands that you wish:

\usepackage[only,llbracket,rrbracket]{stmaryrd}