When to italicize in a math expression?
You may have heard that documents generated in LaTeX are “classy.” It’s true. It was this “elegance” that encouraged me to learn LaTeX; although it wasn’t the only reason for me to continue using this document preparation system. But what is the source of the elegance it adds to your document?
One of the main answers is the consistency of LaTeX for generating mathematical expressions. By generating a document full of equations and symbols in LaTeX, you have the luxury of using many packages to facilitate writing property. For example by using the amsmath package and putting a letter between \( and \) a nice mathematically standard italic letter will be generated. For instance, see the following letters,
Imagine a quantity (I deliberately didn’t use the word variable) that has more than one letter (like Nu, a symbol for the dimensionless Nusselt number, which is very useful in heat transfer and fluid dynamics). When you use \( and \), you will get
As you see, all the letters are italicized.
Now, let’s assume we have two variables N and u that we’d like to calculate their product. To do so, we’ll write
Do you see the problem? Nu cannot be both the multiplication of two variables and the Nusselt number.
What should be italicized?
As you may have guessed, not everything should be italicized in a mathematical expression. But what are the rules? There is a comprehensive standard but as a rule of thumb remember this: any italicized letter is a variable. So, none of the dimensionless numbers, none of the superscripts or subscripts that are not variable and none of the functions’ names. However, there are some exceptions, e.g. the constant e, the imaginary unit i and the function symbol f(x) should be italicized. There are many LaTeX packages like physics that help you follow the convention without too much effort.
The table below depicts the difference between the italicized and non-italicized terms.
This article is short and introductory. So, there are many cases that it does not cover.
More information is available in https://en.wikipedia.org/wiki/Typographical_conventions_in_mathematical_formulae
or