Don’t Start a Sentence With a Mathematical Expression

Don’t start a sentence with a mathematical expression (numerals, variables, or equations). Instead, start every sentence with a word. Consider, for example, that you have a set $A$. It’s confusing to say: “$A$ is …” because the symbol “$A$” looks like the word “A”. Similar confusion can ensue if a sentence ends with a mathematical expression and the next starts with one. For example, “The value of $X$ is $2$. $5$, $6$ and $7$ are numbers.” It looks, at first blush, that $X = 2.5$. When a clause ends with a mathematical expression followed by a comma, avoid placing another mathematical expression immediately after the comma.

There are several ways to avoid starting with a mathematical expression.

• Say the type of the object before the symbol: “The set $A$ is…,” “The expression $\sigma(2 + x)$ represents…”, “The inequality $a^2 \leq b^2$ holds for…”.
• Say the name of the object or short description: “The empty set $\emptyset$ is…”, “The union $A \cup B$…”.
• Reorder the sentence.