# Characteristic function operations

Let $X$ be a set and $A\subseteq X$. If I define the characteristic function of $A$ (relative to $X$) as the function $\chi_A: X\to \mathbb{F}_2$ such that

$$\chi_A(x) = \begin{cases}0; & x\not\in A \\ 1; & x\in A \end{cases}.$$

If $A, B\subseteq X$, then $\chi_A + \chi_B$ and $\chi_A\chi_B$ are both well defined functions from $X \to \mathbb{F}_2$

If $\chi_D = \chi_A + \chi_B$, then $\chi_D\left(x\right) = 1$ if exactly one of $\chi_A\left(x\right), \ \chi_B\left(x\right) = 1$, so $D = A\vartriangle B$.

I am trying to jump to the identity that:

$$C = A\cup B \iff \chi_C = \max\left(\chi_A, \chi_B\right).$$

Why is it necessary to show that $\mathbb{F}_2$ is an ordered field?

• Perhaps I misunderstand your Question. In the conventional definition $\mathbb F_2$ is not an ordered field (it does not have a "positive cone" closed under addition and multiplication). The point of it "being necessary to show" that escapes me. – hardmath Feb 9 '18 at 2:53
• You need an order to be even be able to talk about $\max$ (this is for the "necessary" part of your question). However, as pointed out by @hardmath, $\mathbb{F}_2$ is not an ordered field. Being an ordered field is much stronger than being a field and equipped with an order. IMHO, you should think of the image of characteristic functions living in a Boolean algebra rather than a field. Even though it does not change anything about computations, it will prevent you from looking for an order in $\mathbb{F}_2$ which is not natural. – Clément Guérin Feb 9 '18 at 3:01

Simply because you couldn't have $\max$ in an unordered field.