Use \llbracket
and \rrbracket
from fourier
package (for example) for those doubled bracket symbols.
\documentclass{article}
\usepackage{mathtools}
\usepackage{fourier}
\begin{document}
$ f_{Z} \llbracket x \coloneqq y + z\rrbracket (\sigma) = [ x \mapsto Z ] (\sigma)$
\end{document}

If using the whole fourier
package is too much, importing symbols might be an option: Import one symbol from `fourier`
Here's the version with import of the symbols (code copied from fourier.sty
)
\documentclass{article}
\usepackage{mathtools}
\DeclareFontEncoding{FML}{}{}%
\DeclareFontSubstitution{FML}{futm}{m}{it}%
\DeclareFontEncoding{FMS}{}{}
\DeclareFontSubstitution{FMS}{futm}{m}{n}
\DeclareFontEncoding{FMX}{}{}
\DeclareFontSubstitution{FMX}{futm}{m}{n}
\DeclareSymbolFont{symbols}{FMS}{futm}{m}{n}%
\DeclareSymbolFont{largesymbols}{FMX}{futm}{m}{n}%
\DeclareMathDelimiter{\llbracket}{\mathopen}{symbols}{153}{largesymbols}{133}
\DeclareMathDelimiter{\rrbracket}{\mathclose}{symbols}{154}{largesymbols}{134}
\begin{document}
$ f_{Z} \llbracket x \coloneqq y + z\rrbracket (\sigma) = [ x \mapsto Z ] (\sigma)$
\end{document}

Please note the difference between the regular symbols f
, x
etc. from the screen shots.
fourier
package provides\llbracket
and\rrbracket
stmaryrd
package has\llbracket
and\rrbracket
. I think this site could be useful for you: detexify.kirelabs.org/classify.html.:=
should by typeset not as:=
, but as\coloneq
.bracket
in the comprehensive list of symbols a solution.