amssymbsymbols
The amssymb package has a binary relation symbol named \between, which I have never come across.
What is it used for?
I found out from a compiler error when I tried to define a symbol named that way myself, and I am now wondering whether it is a symbol I could use for my purpose.
Ah well it all depends.
In the default setup as commented in the linked question they are the same symbol but with different mathclass (so different spacing) \bot is a mathord (like an ordinary letter) and \perp is a relation (like <). Relations get more space either side if used between two symbols but in M^{\perp} the mathlist that forms the superscript only has one atom, so there is no additional spacing applied. That means that M^\bot and M^\perp produce identical output.
You could argue that \bot was better as it is naturally a symbol and a relation is not intended here.
Or you could argue that \perp is better as bot refers to a logical notion of bottom/false whereas perp refers to perpendicular which is somehow semantically related to orthogonal.
Or you could argue that they make identical output so it makes no difference and you can use either.
In the Comprehensive List (texdoc symbols) one can find something near to what you want, looking for \independent. Here's a modified version:
\documentclass{article}
\makeatletter
\DeclareRobustCommand{\varamalg}{%
\mathbin{\mathpalette\var@malg\perp}%
}
\newcommand\var@malg[2]{%
\rlap{$\m@th#1#2$}\mkern6mu{#1#2}%
}
\makeatother
\begin{document}
\[
\alpha\varamalg\beta=\alpha\times\{0\}\cup\beta\times\{1\}
\]
\end{document}
Adjust \mkern6mu to suit.
Best Answer
I found it in the Journal of Combinatorial Optimization (2007) 13:217-221, February 14, 2007: A 2-approximation for the preceding-and-crossing structured 2-interval pattern problem. There the expression
\{<,\between\}has been used, as can be seen in the abstract.Regarding intervals,
may stand for a crossing/overlapping relation like < for a precedence order and kind of a subset symbol for inclusion/nesting. See also Extracting constrained 2-interval subsets in 2-interval sets.
\betweenIn such difficult cases, the LaTeX search engine of Springer is a useful tool (now discontinued).