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Hi Yuliang,

On Thu, Jan 06, 2022 at 10:44:40PM +0800, Yuliang Wang wrote:
> In math mode if one inputs `ab` with no space between the two letters ,
> then we get the upright letters `ab`. What is the nature of the string `ab`
> in this case?

"Words" of several normal letters typed in succession are regarded as
operators by TeXmacs,
which makes it easy to type 'sin x' just by typing 's i n space x'.

(See also section 4.2.2 of The Jolly Writer.)

> It's not converted to normal text, as I can tell from the status bar. They
> are still separate math symbols, but typed in upright case (but with no
> `math-up` tag). What's the intention of this face-changing behavior? Of
> course, we can insert a * to get what we want and everything is
> semantically correct.

Yes, if you mean 'a times b', then you should type 'a * b'.

> I don't know about other subjects, but in mathematics one almost never
> represent a variable by more than one letter. So `ab` means nothing but
> `a*b`. Then why not insert a multiplication symbol automatically so that
> the * will not become the most used key when typing math documents. If I
> want an operator named `ab`, then using the "insert operator" is a better
> way?

Well, 'ab' rarely is meant to be an operator, but many other juxtapositions
of two characters often are.
Also, we want you to make an explicit difference between 'f x' (as f applied
to x, 'f space x') and
'a b' (as a times b, 'a * b').

> A somewhat related question is the following. There are 3 methods to type
> $\sin x$ in the math mode.
> 1. Type `sin x`
> 2. Type `\sin x`
> 3. Use toolbar icon `op` $\rightarrow$ `Normal`, type `sin`, exit the tag,
> then type space and x. Of course, here we have more options for the
> typeface.
> The appearance of the three are the same and they are all semantically
> correct. The 3rd approach has an additional tag 'math-up' around each
> letter in `sin`. Which one is the correct approach?

The recommended way is the first one. The second one is for (ex-)LaTeX users.
The third one is overkill, but can be useful for operators in a different
font,
like a 'sans serif' operator.

Best wishes, --Joris

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• Some thoughts and questions about math symbols, multiplication and operators, Yuliang Wang, 01/06/2022
  • Re: Some thoughts and questions about math symbols, multiplication and operators, TeXmacs, 01/06/2022

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