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I understand that this is not strictly a TeX question but I have recently been asked to correct an equation presented as the first example in the image below to that of the second example i.e., to use the thousands separator. I compromised by typesetting the third example with a \thinspace as the thousands separator.

enter image description here

All three types are allowed in SI units. What would you recommend? Is there a canonical way?

Edit

I came across a paper by Knuth, where large number coefficients are used.

enter image description here

He clearly is in favour of not using any separators or spaces. This fact, besides his own papers is corroborated by an anecdote described in Mathematical Writing (page 53), regarding an article he submitted to the ACM.

[...] where Don wrote 1000000 they substituted 1,000,000. Don objected that although this might be justifed in text, his use is perfectly OK in a formula. Well then, they replied, write 106 Fine, said, Don, but what do I do when the number is 1234567? The IEEE standard here is to insert spaces, thus: 1 234 567. Don doesn't like this in formulae, but agrees that it may be useful in a high precision context, such as numerical tables.

For me that settles it!

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from mathematical typesetting completely without additional space. –  Herbert Jan 26 '11 at 19:16
    
@Herbert Traditionally we would not separate them, hence my choice. The counter-argument was that there is no valid reason not to and that it improves readability. Steven G. Krantz in Handbook of Typography for the Mathematical Sciences is silent about this. –  Yiannis Lazarides Jan 26 '11 at 19:23
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If you were to go for the middle version (commas), at least use {,} rather than the bare comma. That will fix the rather glaring problem with spacing. Personally, I lean toward the thin space solution, even in mathematics, at least if the numbers have nine digits or more. But I might be tempted to break with tradition and put them in groups of five rather than three. –  Harald Hanche-Olsen Jan 26 '11 at 19:28
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@Yannis: you want to show an approximation of a function, the reason why it is not important for the reader how large a number may be, important is only how the number is build; the definition of erf^{-1} as a sum is more of interest to the reader –  Herbert Jan 26 '11 at 19:35
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Looking through the maths course I'm currently doing (OU M373: Optimisation), large numbers are divided up in SI style 0.123 456, with what look like thin spaces. However, that is in tables rather than in equations, at least that I've found so far. –  Joseph Wright Jan 26 '11 at 20:31

2 Answers 2

up vote 11 down vote accepted

If you were to go for the middle version (commas), at least use {,} rather than the bare comma. That will fix the rather glaring problem with spacing. Personally, I lean toward the thin space solution, even in mathematics, at least if the numbers have nine digits or more. But I might be tempted to break with tradition and put them in groups of five rather than three.

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Your question rely upon your country's mathematics typesetting triditional habbit, and it is not a TEX problem. Anyway, You could input

\frac{\pi^2}{123\thousandsymbol 456}

to get diffrence output easily. If your codes had been finished, the better way (as far as I know) to modify them is using regular expression, adding the command inside digits (on reverse order).

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10  
If it is needed, numprint or siunitx may be a better choice. –  Leo Liu Jan 27 '11 at 8:15
    
You're right. I hadn't read the ealier comments yet before i submitted. –  luochaodong Jan 27 '11 at 8:54

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