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I don't know what it is even called so I cannot really type it out properly, sorry. I've seen things like $X$|_|$[0,1)$. The symbol in question looks like $\cup$ but without curvy, round bits, its like the bottom half of a square.

I don't know how else to describe it and it look bizarre.

Can anyone tell me? Thank you

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    $\begingroup$ Is it $\sqcup$? Or $\amalg$? $\endgroup$ Commented Feb 17, 2016 at 15:39
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    $\begingroup$ $\sqcup$ is typically used to denote disjoint union. Meaning, you take the union of copies of the sets in questions but without common elements. It is standard to "make the sets disjoint" by interpreting $A\sqcup B$ to mean $(A\times\{0\})\cup (B\times\{1\})$. Similarly for unions of more than two sets. $\endgroup$ Commented Feb 17, 2016 at 15:41
  • $\begingroup$ Also referred to as the "coproduct" in category theory. See Lang's Algebra. $\endgroup$ Commented Feb 17, 2016 at 15:49
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    $\begingroup$ By the way you can use detexify.kirelabs.org/classify.html to look up Latex symbols from a drawing of them. $\endgroup$ Commented Feb 17, 2016 at 16:43
  • $\begingroup$ Flat bottom union (for google) $\endgroup$ Commented Apr 5, 2024 at 16:33

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$A\sqcup B$ is just $A\cup B$, but it gives the information that $A$ and $B$ are disjoint, i.e. $A\cap B=\emptyset$.

In probability theory, this notation can also be used for independence. For example, $X\sqcup Y$ means that the random variables $X$ and $Y$ are independent.

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  • $\begingroup$ That's the notation I'm looking for, thanks! $\endgroup$ Commented Feb 17, 2016 at 18:51
  • $\begingroup$ Not exactly, sometimes we define the disjoint union of $A$ and $B$, i.e., $A\sqcup B=\{(a,0)\mid a\in A\}\cup\{(b,1)\mid b\in B\}$ which is well defined no matter $A$ and $B$ are disjoint. $\endgroup$ Commented Oct 27, 2021 at 4:15

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