AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Typography and Capitalization
Mathematical Composition

In general, variables, unknown quantities, and constants (eg, x, y, z, A, B, C ) are set in italics, whereas units of measure (eg, kg, mL, s, m), symbols (including Greek characters [see 16.0, Greek Letters]), and numbers are set roman. In addition, subscripts or superscripts used as modifiers are set roman:

Note: Do not display forms of statistical analysis as subscript, such as P_interaction < .001. Better: P < .001 for interaction (see 19.0, Study Design and Statistics).

Arrays (A) and vectors (V) should be set boldface. Mathematical functions, such as sin, cos, ln, and log, are set roman.

For equations that are set off from the text, the words and letters should be set roman and the equation should be capitalized by the same rules that apply to titles (see 10.2, Titles and Headings).