How can I have a big $[\![ \overrightarrow{\mathcal A} ]\!]$?

Question

I would like to have the double square-brackets that are scaled up according to the symbol inside [\![ \overrightarrow{\mathcal A} ]\!]. I tried \left [\![ \overrightarrow{\mathcal A} ]\! \right], but to no avail. How can I do so?

$\left [\!\left[ \overrightarrow{\mathcal A} \right]\!\right]$ will separate the double bracket (as in below image). I want to keep them share the same horizontal edge:

Answer 1

OK your solution is this. Use the stmaryrd package that has useful bracket and parentheses styles.

So the code should be this $\left\llbracket\overrightarrow{\mathcal A} \right\rrbracket$. This allows the brackets to share common edges as well as scale up.

This is adapted from here: Double square brackets with slight modification for this solution.

Answer 2

The double brackets are \lBrack and rBrack in several packages, including unicode-math, stix, stix2 and fdsymbol. A convenient way to use them is in a paired math delimiter from mathtools.

\documentclass{article}
\usepackage{mathtools}
\usepackage{unicode-math}

\DeclarePairedDelimiter\Brackets{\lBrack}{\rBrack}

\begin{document}
\[ \Brackets*{\overrightarrow{\mathcal A}\;}
\]
\end{document}

You can manually specify the size of the brackets with, e.g., \Brackets[\big]. Here, I added a bit of space on the right to compensate for the arrow smashing into the right delimiter, which is not a great solution.

You could also adjust the position of the arrow with \skew:

\documentclass{article}
\usepackage{mathtools}
\usepackage{unicode-math}

\DeclarePairedDelimiter\Brackets{\lBrack}{\rBrack}
\newcommand\Aaarow{\skew{-2}\overrightarrow{\mathcal{A}}}

\begin{document}
\[ \Brackets*{\Aaarow}
\]
\end{document}

Or you could use a font whose \overrightarrow does not extend as far, such as STIX Two Math.