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I want to type the following symbol (for logical equivalence):
enter image description here

Some editors have the command \DashvDash available, but SE's MathJax doesn't.
Alternatively, the symbol can be defined via \reflectbox{\vDash}\!\vDash, but this requires the package graphicx which isn't included either.

How can I render the above symbol in SE's MathJax?

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  • $\begingroup$ Doesn't $\equiv$ suffice? \equiv $\endgroup$
    – amWhy
    Commented Jun 22, 2020 at 15:54
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    $\begingroup$ No, some sources reserve $\equiv$ for syntactic identity (and I also prefer the analogy to $\vDash$ over a completely different looking symbol). $\endgroup$
    – Natalie Clarius
    Commented Jun 22, 2020 at 16:16

2 Answers 2

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The commands \DashvDash and \dashVdash in LaTeX are part of the stix package. As far as I can tell, they are not available in MathJax. However, you could use Unicode. (These symbols can be found in the Unicode block Miscellaneous Mathematical Symbols-A.) Or you could simply copy-paste them when needed.

  • $⟛$ and $⟚$ produces $⟛$ and $⟚$
  • $\unicode{x27DA}$ and $\unicode{x27DB}$ produced $\unicode{x27DA}$ and $\unicode{x27DB}$

You might also use \mathrel to get the required spacing. (In the stix package in LaTeX, this is a relation symbol.) Compare $x \mathrel{⟛} y$ and $x ⟛ y$. (Obtained by $x \mathrel{⟛} y$ and $x ⟛ y$.)

If you have a post where you use this symbol often, you might want to create a shortcut. For example, after including $\newcommand{\DvD}{\mathrel{\unicode{x27DB}}}$ you can then simply use in the post \DvD whenever you need this command. (I have avoided using the standard name from the stix package on purpose - it seems better not to redefine standard macros. We do not know whether these commands might be implemented in MathJax at some point in the future.)

Example: $\newcommand{\DvD}{\mathrel{\unicode{x27DB}}}\DvD$.

I would have included some examples similar to the actual usage of this symbol - but I am not familiar with it.

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How does $=\hspace{-5pt}|\hspace{-.5pt}|\hspace{-5pt}=$ look?

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    $\begingroup$ Neat! Combining with Martin Sleziak's answer to make it a relation, this can be defined as a new command at the start of a post, to save excess typing: $\newcommand{DashvDash}{\mathrel{=\hspace{-5pt}|\hspace{-.5pt}|\hspace{-5pt}=}}$ So defining silently: $\newcommand{DashvDash}{\mathrel{=\hspace{-5pt}|\hspace{-.5pt}|\hspace{-5pt}=}}$, and using it here: $a\DashvDash b$. $\endgroup$
    – user1729
    Commented Jun 22, 2020 at 17:48

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