# How would you notate that something is not in the domain of discourse?

For example, saying that something is specifically not in the domain of integers (not sure how to write the 'N' symbol).

• Like $a\notin\Bbb N$? – Lord Shark the Unknown Apr 19 at 14:42
• To produce $\mathbb{N}$, type $\mathbb{N}$. This tutorial explains how to typeset mathematics on this site. – N. F. Taussig Apr 19 at 14:44
• Thanks for the help. I have realised I was being silly. I was thinking of something else and so I skipped over the fact that 'a∉N' is what I am looking for - I can't explain why, I am just silly. – Jake Jackson Apr 19 at 14:51
• It may be useful to remember that we can use something like $a \in \mathbb{Q}\backslash \mathbb{Z}$ if we want a non-integer rational number. – Ertxiem Apr 19 at 16:05

Traditionally, the integers are denoted by $$\mathbb{Z}$$ which in Latex is written $\mathbb{Z}$. By enclosing Z in \mathbb{}, you are telling Latex to write Z in the math blackboard bold font, which is the standard font for sets of numbers. The natural numbers are denoted by $$\mathbb{N}$$, i.e. $\mathbb{N}$.
Mathematicians use an epsilon symbol to mean "is an element of." That is, $$x \in \mathbb{Z}$$ is read as "$$x$$ is an element of $$\mathbb{Z}$$." The Latex command for this epsilon is \in.
You want to say that something is not in the integers. To negate a statement, mathematicians often put a slash through a symbol. The statement $$x \not\in \mathbb{Z}$$ is read as "$$x$$ is not an element of $$\mathbb{Z}$$." The Latex command for this is $x \not\in \mathbb{Z}$. In general, prefixing a symbol with '\not' in Latex will put a slash through it.