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#1




upright Greek letters for constants as opposed to variables
Some journals require upright Greek letters for constants as opposed to
variables, for example $\upi$ when used to denote 3.14159... as opposed to a variable ($\pi$ is sometimes used to denote parallax in astronomy, for instance). (Some journals define \upi as "upright pi", \upi as "upright i", and so on.) I certainly agree that LABELS should be upright (though they are usually Latin not Greek) and not italic to distinguish them from variables, e.g. $T_{mathrm{eff}}$ for effective temperature or $\rho_{textrm{g}}$ for gas density, say, as opposed to $G_{\mu\nu}$ where $\mu$ and $\nu$ are not constants but variables. And it is not just Greek letters. For example, e for the Euler number or i for the square root of 1 should also not be in math italic, to distinguish them from variables. I tend to agree with that as well. Also, units should be upright, e.g. 5 m and not $5m$ for 5 metres. On the other hand, I have never seen the gravitational constant $G$, which is even by definition a constant and not a variable, written upright. Ditto for the Hubble constant $H$ and so on. Or is there a difference between mathematical constants and physical constants? Perhaps because standard (La)TeX provides Greek letters only in math italic, upright Greek letters are less common than upright Latin letters, even when used in the same way (labels, units, symbols which are not variables). When writing for a specific journal, one usually has to follow the house style. However, if there is no rule, I prefer to do what is generally deemed to be correct. What is generally deemed to be correct here? 
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#2




upright Greek letters for constants as opposed to variables
On 20200427 00:05, Phillip Helbig (undress to reply) wrote:
Some journals require upright Greek letters for constants as opposed to variables, for example $\upi$ when used to denote 3.14159... as opposed to a variable ($\pi$ is sometimes used to denote parallax in astronomy, for instance). (Some journals define \upi as "upright pi", \upi as "upright i", and so on.) .... Or is there a difference between mathematical constants and physical constants? The principle probably comes from pure mathematics, but even there, historically, they have not been available, just as well certain styles, such as bold italic, for the simple reason that it was expensive in led typesetting to keep them. Some journals would though have them, and one could mark up the manuscript to get the right one. Unicode changed that by adding those as characters (code points), in addition adding some styles that are not properly semantic, like sans serif and monospace variations. (The sans serif style is used for tensors by some engineers, but I have found no example in mathematics, physics or computer science of that.) [[Mod. note  Checking a few generalrelativity textbooks, I see that Misner, Thorne, & Wheeler "Gravitation" (W.H. Freeman, 1973) uses sans serif for tensors and differential forms, but none of the other books I checked do this.  jt]] So when those are available, one can experiment with adhering to this principle, upright for constants and italic/slanted for variables. In some cases it may not be immediately clear what to use: Unicode unifies the upright style with the original language letters. TeX, by contrast, translates them automatically to italic in math mode. For Greek, it only has the slanted styles, and in addition not the uppercase letters that look like the Latin. Perhaps because standard (La)TeX provides Greek letters only in math italic, upright Greek letters are less common than upright Latin letters, even when used in the same way (labels, units, symbols which are not variables). So I switched typing these mathematical styles directly in the input file, and the fastest way to do that, both to implement and use, I found is to use text substitutions. Then the LaTeX unicode math package was insufficient, so I switched to ConTeXt. When writing for a specific journal, one usually has to follow the house style. However, if there is no rule, I prefer to do what is generally deemed to be correct. What is generally deemed to be correct here? You will have to experiment with it a bit. 
#3




upright Greek letters for constants as opposed to variables
On 20200516 05:09, Mod wrote:
Unicode changed that by adding those as characters (code points), in addition adding some styles that are not properly semantic, like sans serif and monospace variations. (The sans serif style is used for tensors by some engineers, but I have found no example in mathematics, physics or computer science of that.) [[Mod. note  Checking a few generalrelativity textbooks, I see that Misner, Thorne, & Wheeler "Gravitation" (W.H. Freeman, 1973) uses sans serif for tensors and differential forms, but none of the other books I checked do this.  jt]] They use bold sansserif when not having indices, but switches to nonbold serif, that is, plain italic, when having indices on the same object. In mathematics, like in differential geometry, one uses serifs and not switching styles, like between plain and bold, allowing the styles to be used for more different types of objects objects. 
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