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Wikipedia lists several dot characters in Unicode. These are the ones that are ambiguous to me:

  • interpunct, middle dot (·) · · U+00B7 "midpoint (in typography)"
  • dot operator (⋅) ⋅ ⋅ U+22C5
  • bullet operator (∙) ∙ U+2219 "(mathematics)"
  • bullet (•) • • U+2022 "black small circle" "often used to mark list items"

I'm mostly interested in the Unicode characters, but Math.SE told me to try here, and LaTeX likewise has:

  • \cdot
  • \cdotp
  • \bullet
  • \textbullet

For example:

  • Wikipedia shows multiplication as both "x ⋅ y" ("dot operator") and "x ∙ y" ("bullet operator") in the same paragraph.
  • WP List of mathematical symbols uses "middle dot" for multiplication.
  • WP Multiplication article uses \cdot for scalar multiplication.
  • WP says the matrix dot product should be written using the "bullet operator" character, like "ab".
  • The WP Dot product article uses the LaTeX \cdot character for dot products.
  • Wikipedia shows a raised decimal point example "£21·48", which uses the "middle dot" character.
  • Wikipedia notates chemistry hydrates like "CuSO4 · 5H2O", which uses the "middle dot" character.
  • Combined units can also be written with a dot, like "N·m". Wikipedia uses a "middle dot" for this, not the "dot operator".
  • Characters in SI notations has a paragraph on combined units, seeming to say that dot operator is preferred for this, like "N⋅m".
  • Wolfram Alpha considers \cdot and · to be equivalent, as well as \bullet and •. Valid?

Of course it's pedantic, they all look the same, and the meaning can always be assumed from context, but I'd like to know, once and for all, which Unicode/HTML and LaTeX characters are semantically correct in each application?

(If there are other examples I left out, feel free to edit the question and add them.)

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Make the SuperUser logo with the Bullet: [•} –  YatharthROCK Sep 23 '12 at 17:10

2 Answers 2

The following is my personal view on this matter. I have not seen hard rules anywhere.

  • Normal mathematics

The \cdot is a multiplication symbol. The following are all equivalent

enter image description here

It is useful in thermo-fluid dynamics fromulas where there are a lot of multi character values such as Reynolds number, Prantl number, etc.

enter image description here

Compound SI units (see siuntitx package) is also multiplication

enter image description here

For vector-tensors it is element wise multiplication

enter image description here

  • Chemical formula

I don't know much about chemical notation (ask Joseph Wright)

  • Decimal marker

The center dot as decimal marker is only found in very old books or non-scientific litterature. Because it can be confused with a multiplication sign is is not used in scientific literature any more.

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2  
Some cultures swap and use "." for multiplication and "·" for the decimal, apparently. –  endolith May 28 '11 at 4:10
2  
Not just other cultures, but other time frames of Western European culture did so too. –  frabjous Jun 7 '11 at 16:44
    
Do hard rules exist? They might be specified for Unicode or for TeX, for instance. Even house rules for math typesetting would be nice. –  endolith Oct 24 '11 at 21:11
    
@Danie Els: What font did you use? Your exported pictures look extremely good! –  ralfix Mar 1 '13 at 20:56
    
@ralfix: This was the math fonts of MicroPress Charter, a commercial font. The charter option of mathdesign looks more or less the same. –  Danie Els Mar 2 '13 at 3:22

As the Unicode project seeks to determine the semantics of the glyphs rather than their graphical representation, this question is quite interesting.

Although I agree that all of these “bullets” could theoretically look the same (and one wouldn’t need to care about this issue at all), the semantic background can—at least to some degree—be inferred by the official Unicode charts:

Since two of the four symbols, which are subject of the discussion, are placed in Unicode Blocks named “punctuation” (see below), I would advise against using them in mathematical context. In my opinion, the annotation “= midpoint (in typography)” for the 00B7 · MIDDLE DOT symbol speaks for itself and I think one could use it in typesetting poems, for example, to mark stanzas or as a substitute for the regular space.

From my point of view, the 2022 • BULLET should be used as a list symbol exclusively.

As I think it, the 22C5 ⋅ DOT OPERATOR is the mathematical version of 00B7 · MIDDLE DOT and 2219 ∙ BULLET OPERATOR of 2022 • BULLET, respectively, when it comes to appearance. One will probably get the most “regular-looking” result with using the 22C5 ⋅ DOT OPERATOR for regular (scalar) multiplication and the 2219 ∙ BULLET OPERATOR for any other (user-defined) operation on other objects.
(See the image prepared by Danie Els.)

This can be done, from my point of view again, because the Unicode standard does not define this operator to represent any mathematical operation. Interestingly, for the 2218 ∘ RING OPERATOR symbol, the annotation does just that: “= composite function”. So I would say that semantically you are on the safe side, so to speak, as maybe the Unicode Consortium has spoken to mathematicians who told them that the dot can be—and which is being—redefined to meet the mathematician’s need. (This is very interesting when speaking about abstract algebraic structures, such as fields and vector spaces, when one uses only abstract operations.)

Since, your query for “house rules for mathematical typesetting” is not so far off, forasmuch I have experienced that in my math lectures, every professor would define these symbols at the beginning of the semester and there were differences throughout the lectures, of course.

TL;DR:
Use 22C5 ⋅ DOT OPERATOR for multiplication and 2219 ∙ BULLET OPERATOR for own operations. This is only my opinion.


Excerpts from Unicode specifications

(I think the arrows indicate references to similarly-looking rather than to semantically similar symbols.)

Block “C1 Controls and Latin-1 Supplement”, Section title “Latin-1 punctuation and symbols”

00B7 · MIDDLE DOT
= midpoint (in typography)
= Georgian comma
= Greek middle dot (ano teleia)
→ 0387 · greek ano teleia
→ 16EB ᛫ runic single punctuation
→ 2022 • bullet
→ 2024 . one dot leader
→ 2027 ‧ hyphenation point
→ 2219 ∙ bullet operator
→ 22C5 ⋅ dot operator
→ 2E31 ⸱ word separator middle dot
→ 2E33 ⸳ raised dot
→ 30FB ・ katakana middle dot

Block “General Punctuation

2022 • BULLET
= black small circle
→ 00B7 · middle dot
→ 2024 . one dot leader
→ 2219 ∙ bullet operator
→ 25D8 ◘ inverse bullet
→ 25E6 ◦ white bullet

Block “Mathematical Operators

2219 ∙ BULLET OPERATOR
→ 00B7 · middle dot
→ 2022 • bullet
→ 2024 . one dot leader

22C5 ⋅ DOT OPERATOR
→ 00B7 · middle dot

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4  
what i really don’t get about the unicode consortium is that they include every crap, yet recommend to use »’« (’) as typographic apostrope. wtf? –  flying sheep Nov 9 '12 at 16:46
    
Yes, I agree, given that they normally are very meticulous about separating the semantic part from the graphical representation, this is very off. –  brian-ammon Nov 10 '12 at 16:02

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