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My paper is accepted at MNRAS and in the stage of editing.

There is a section where I wrote an equation using the Greek letter π as to indicate the mathematical constant π.

The editor wrote:

Author: When used to mean the numerical value 3.14, the Greek letter pi must always be written in roman. Please check all the notations throughout the text carefully

I think this request is canned, so I doubt there is any typo or similar here.

What exactly is π in roman? Shouldn't it be written in Greek? Is it a mistake of the editor or a misunderstanding from my side?

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    The discussion about the motivations of this convention, and whether it's good or bad, has been moved to chat. Please read this FAQ before posting another comment.
    – Massimo Ortolano
    Dec 5 '20 at 11:23
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They are referring to the upright version of pi. In the picture below, you can find on the left an italics version and on the right an upright one.

enter image description here

If you're using LaTeX, you can obtain the upright pi with the upgreek package and the command \uppi, see this Q&A on TeX SE.

More details on this convention can be found in the IUPAC Green book, which follows the ISO/IEC 80000 standard. From p. 7 of the linked document:

The overall rule is that symbols representing physical quantities or variables are italic, but symbols representing units, mathematical constants, or labels, are roman.

And from p. 8:

The symbols π (3.141 592. . .), e (2.718 281. . . , base of natural logarithms), i (square root of minus one), etc. are always roman [...]

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    I'll take this opportunity to flag up a possible source of subtler confusion: in the TeX/LaTeX user community, the word "roman" is used specifically to describe an upright font with serifs; but in the ISO 80000 series standards, the word "roman" is used to describe any upright font, regardless of whether or not it has serifs. Dec 3 '20 at 11:22
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    Also in many LaTeX fonts, while the roman (upright) and and italic letters are distinct from each other, the upright pi can still look rather italic. Compare the right hand illustration, upright but with a sloping left stroke and a hook on the top, to the last paragraph of the answer in which (sans serif) pi looks like a table. This means it's common not to bother if pi only means what we're used to from school. In physics we're normally good about getting our "e"s upright, but lazy enough to just use \pi even if it should really be upright
    – Chris H
    Dec 4 '20 at 7:15
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    @ChrisH Yes, in fact I'm quite fussy about having i and e upright in my papers, but less so when it comes to pi, also because certain journal LaTeX classes are not easily compatible with upgreek.
    – Massimo Ortolano
    Dec 4 '20 at 7:52
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    Apparently upgreek not only changes the style of π, but also changes the typeface from Computer Modern Math Italic to Euler Math. Unless all your math is in Euler, this looks ugly. The documentation for the isomath package has several suggestions that retain consistency, the simplest of which is to use the mathdesign package, which defines the macro \piup with a consistent math+text typeface.
    – JeffE
    Dec 4 '20 at 17:12
  • @JeffE Interesting, didn't know that. Probably, every journal class would need a specific solution, and the publisher will likely use proprietary fonts anyway.
    – Massimo Ortolano
    Dec 4 '20 at 17:38
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They mean that it shouldn't be written in italics.

Basically, they weren't referring to the Roman and Greek civilizations, but to typefaces.

Quoting from the Encyclopedia Brittanica:

Roman, in printing, one of the three major typefaces in the history of Western typography (the others being italic and black letter, or Gothic) and, of those three, the face that is of the greatest importance and the widest use.

So, basically, by specifying that pi should be roman, they were specifying that it should be written using a particular sort of typeface, without using italics. Some of the other answers have talked about how to do so using LaTeX, but since none of them have explicitly stated what "roman" means in this context, I figured I'd write an answer explaining it.

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    And, when referring to the Roman alphabet, one can rather speak of the Latin script, so adding another measure of distinction from the Roman typeface. Dec 4 '20 at 20:19
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Specifically at MNRAS, they actually provide an upright (i.e., not italic) version of π in their LaTeX class. To cite the manual:

There are several options which can be added to the document class line like this:
\documentclass[option1,option2]{mnras}
The available options are:

  • ...
  • useAMS– adds support for upright Greek characters \upi,\umuand, \upartial (π,μ and ∂). Only these three are included, if you re-quire other symbols you will need to include the amsmath or amsymb packages

I believe the request for the upright π can be inferred from their General Instructions:

Mathematics
(...) Differential d, complex i, exponential e, (...) are roman (not italic).

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