Mathematical Expressions

Most style manuals do not cover mathematical style well. CMS has some good suggestions in Chapter 12, but is insufficient. Another reference source is N.J. Higham, The Handbook of Writing for the Mathematical Sciences, Society for Industrial and Applied Mathematics, 1998. Although this book is largely about how to write mathematics clearly, it also has some suggestions about math typography and other stylistic matters.

Miscellaneous math style issues

If authors use punctuation after mathematical expressions, including displayed equations, leave it in (or revise as necessary).

• Include a space on either side of multiplication signs, equals signs, and other operators when they are at full size, for example, 100 × 100 matrix, and a + b = c. MathType automatically inserts thin spaces around these symbols. In subscripts and superscripts, omit these thin spaces unless the formula becomes confusing without them.
• If line breaks are needed in equations that appear in a paragraph, they should come after a plus sign, equals sign, or similar character.
• Characters with overbars must be set in MathType. If an article has many symbols with overbars or other symbols that make typesetting difficult, consider asking the author if there is an easier alternative.
• Equation numbers are put in parentheses to the right of displayed formulas. They should not be boldface. Generally speaking, only formulas called out in the text need to be numbered.
• Use italic type for lowercase Greek variables; do not use italic type for uppercase Greek variables.
• Variables that denote vectors are set boldface, not italic. Do not use the small arrows above the variable to denote vectors.
• Set the label that denotes a matrix in italic, for example, A.
• If the vectors are simply mathematical and do not represent physical quantities and direction (that is, they do not need to be clearly differentiated from scalars), boldfacing is less important, and you can use lightface italic type if the author has styled them that way.

If the author consistently uses another scheme, consider going along with it.

Equation formatting guidelines

Display equations can be broken down a number of ways to fit within a column. First, note that each line of an equation must be aligned to either a relation symbol or a binary operator in the first line of the equation (if one exists). These symbols work in a hierarchy: Relation symbols are aligned with other relation symbols, binary operators are aligned with other binary operators, and binary operators are indented from relation symbols. For example:

If there is only one relation symbol or binary operator in an equation, break the equation after the relation symbol or binary operator and indent the second line slightly:

If there is room, align the equation like this:

If a display equation can't be centered, the first line can be made flush left to the column to allow more room for the following lines of the equation to fit. If an equation number won't fit to the right of an equation, it is acceptable to have the equation number fall one line below the equation and to keep it right-justified to the column.