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I'd like to use a symbol to denote the inside of a triangle inside mathematical formulas in an answer I'm writing for Mathematics Stack Exchange.

A natural candidate is a triangle with a dot inside it: ◬. How can I typeset this symbol in MathJax?

It's available in Unicode at code point U+25EC, however when I use the unicode symbol inside a MathJax formula, the symbol's size is considerably smaller than the normal symbol for a triangle whose command is \triangle: $\unicode{x25ec}$ typesets as $\unicode{x25ec}$ whereas $\triangle$ typesets as $\triangle$. I'd like the dotted triangle to have the same size as the symbol that is typeset by the command \triangle.

The dotted triangle symbol is also available as the command \trianglecdot of the LaTeX package stix, but I don't know how to import this package to a Mathematics Stack Exchange answer, and even if I could, it would likely simply resolve to the above-mentioned unicode symbol.

I'm also open to suggestions of alternative symbols.

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    $\begingroup$ Try overlaying symbols. $\endgroup$
    – user1082389
    Commented Jul 13, 2022 at 7:26
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    $\begingroup$ On my computer, the two triangles $◬$ and $\triangle$ are about the same size, even in different math renderers html-css: i.sstatic.net/AbTs2.png and common html: i.sstatic.net/asRvV.png . Of course this is because the unicode symbol used depends on the fonts installed $\endgroup$
    – Calvin Khor
    Commented Jul 13, 2022 at 8:42
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    $\begingroup$ On the suggestion front, you could consider that \mathring A$\mathring A$ is used to denote the topological interior of a set $A$, so why not \mathring\triangle$\mathring\triangle$? $\endgroup$
    – Calvin Khor
    Commented Jul 13, 2022 at 8:49
  • $\begingroup$ $\text{int}(\triangle)$ seems best to me as $\text{int}(S)$ is a classical notation for interior of a set. (command is ´\text{int}(\triangle))´) $\endgroup$
    – Surb
    Commented Jul 13, 2022 at 21:47

4 Answers 4

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Expanding on Be Kind's comment above.

You can use the \rlap command to overlay one character on another:
$\rlap{\,\,\cdot}\triangle$ gives $\rlap{\,\,\cdot}\triangle$

I'm not sure how or if it scales, but $\large \rlap{\,\,\cdot}\triangle$ does make it bigger: $\large \rlap{\,\,\cdot}\triangle$

The \,s are used to create space so the dot moves to the right. You can experiment with different spacings until the dot appears centered in the triangle.

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    $\begingroup$ The precise amount of left/right spacing needed is unfortunately device dependent; see for instance Why does MathJax render differently on different devices? This is not a new problem, but I have discovered a new way to overlap, which trades the horizontal inaccuracy for vertical inaccuracy; sometimes better.\overset{\cdot}{\smash\triangle}$\overset{\cdot}{\smash\triangle}$. See also $\overset{\bullet}{\smash\triangle}\overset{\circ}{\smash\triangle}\overset{\blacksquare}{\smash\triangle}\overset{\blacktriangle}{\smash\triangle}$ $\endgroup$
    – Calvin Khor
    Commented Jul 13, 2022 at 12:45
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A possible alternative symbol: a black triangle. $\blacktriangle$ typesets as $\blacktriangle$. Compare the black triangle with the triangle that is typeset by \triangle: $\blacktriangle\ \triangle$. Its size can be enlarged with \large thus: ${\large\blacktriangle}\ \triangle$.

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I would like to suggest something for the

I'm also open to suggestions of alternative symbols.

$\text{int}(\triangle)$ seems good to me as $\text{int}(S)$ is a classical notation for interior of a set. Command is \text{int}(\triangle))

Actually, IMHO, mathematically best is:

Let $T$ be a triangle and let $\text{int}(T)$ denote its interior with respect to the Euclidean topology.

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    $\begingroup$ $\DeclareMathOperator{\intt}{int}$If the operator is being used many times in the post, one can also write $\DeclareMathOperator{\intt}{int}$ at the start of the post, and then just use $\intt(\triangle)$. This will produce the same output: $\intt(\triangle)$. $\endgroup$
    – user1082389
    Commented Jul 14, 2022 at 5:37
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    $\begingroup$ Thanks. This is not adequate for my purposes, since in my use case the triangle symbol is used in the standard way it is used in geometry as an operator not a set, thus: $\triangle ABC$ for the triangle with vertices $A$, $B$, and $C$. $\endgroup$
    – Evan Aad
    Commented Jul 14, 2022 at 6:03
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    $\begingroup$ it seems that the natural suggestion would then be $\operatorname{int}(\triangle ABC)$. $\endgroup$
    – Calvin Khor
    Commented Jul 14, 2022 at 9:34
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$\triangle\hspace{-7pt}\bullet$

$$\triangle\hspace{-7pt}\bullet$$

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