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.
All three types are allowed in SI units. What would you recommend? Is there a canonical way?
I came across a paper by Knuth, where large number coefficients are used.
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!