A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Equations and formulas
Numbers
Part III. Style 20 spelling

Numbers in equations and formulas are always given as numerals, regardless of the general rule (see 23.1.1). For detailed guidance on presenting mathematical expressions, see chapter 14 of the Chicago Manual of Style, 15th edition (2003).