\section{Structures}

\definition{}
A \textit{universe} is a set of meaningless objects. Here are a few examples:
\begin{itemize}
	\item $\{a, b, ..., z\}$
	\item $\{0, 1\}$
	\item $\mathbb{Z}$, $\mathbb{R}$, etc.
\end{itemize}

\definition{}
A \textit{structure} consists of a universe $U$ and a set of symbols. \par
A structure's symbols give meaning to the objects in its universe.

\vspace{2mm}

Symbols come in three types:
\begin{itemize}
	\item Constant symbols, which let us specify specific elements of our universe. \par
		Examples: $0, 1, \frac{1}{2}, \pi$
		\vspace{2mm}

	\item Function symbols, which let us navigate between elements of our universe. \par
		Examples: $+, \times, \sin{x}, \sqrt{x}$
		\vspace{2mm}

	\item Relation symbols, which let us compare elements of our universe. \par
		Examples: $<, >, \leq, \geq$ \par
		\vspace{2mm}
\end{itemize}

The equality check $=$ is \textbf{not} a relation symbol. It is included in every structure by default.

\vspace{3mm}


\example{}
The first structure we'll look at is the following:
$$
	\Bigl( \mathbb{Z} ~\big|~ \{0, 1, +, -, <\} \Bigr)
$$

\vspace{2mm}

This is a structure with the universe $\mathbb{Z}$ that contains the following symbols:
\begin{itemize}
	\item Constants: \tab $\{0, 1\}$
	\item Functions: \tab $\{+, -\}$
	\item Relations: \tab $\{<\}$
\end{itemize}

\vspace{2mm}

If you look at our set of constant symbols, you'll see that the only integers we can directly refer to in this structure are 0 and 1. If we want any others, we must define them using the tools this structure offers.

\vspace{1mm}

To \say{define} an element of a set, we need to write a sentence that is only true for that element. For example, if we want to define 2 in the structure above, we could use the sentence $\varphi(x) = [1 + 1 = x]$. \par
Clearly, this is only true when $x = 2$.


\problem{}
Define $-1$ in $\Bigl( \mathbb{Z} ~\big|~ \{0, 1, +, -, <\} \Bigr)$.

\vfill
\pagebreak

Let us formalize what we found in the previous two problems. \par

\definition{}
A \textit{formula} in a structure $S$ is a well-formed string of constants, functions, and relations. \par

\vspace{2mm}

You already know what a \say{well-formed} string is: $1 + 1$ is fine, $\sqrt{+}$ is nonsense. \par
For the sake of time, I will not provide a formal definition. It isn't particularly interesting.

\vspace{2mm}

A formula can contain one or more \textit{free variables.} These are denoted $\varphi{(a, b, ...)}$. \par
Formulas with free variables let us define \say{properties} that certain objects have. \par

For example, $x$ is a free variable in the formula $\varphi(x) = [x > 0]$. \par
$\varphi(3)$ is true and $\varphi(-3)$ is false. \par

\vspace{2mm}

This \say{free variable} notation is much like the function notation you are used to: \par
$\varphi(x) = [x > 0]$ is similar to $f(x) = x + 1$, since the values of $\varphi(x)$ and $f(x)$ depend on $x$.


\definition{Definable Elements}
Say $S$ is a structure with a universe $U$. \par
We say an element $e \in U$ is \textit{definable in $S$} if we can write a formula that only $e$ satisfies.


\problem{}
Define 2 in the structure $\Bigl( \mathbb{Z^+} ~\big|~ \{4, \times \} \Bigr)$. \par
\hint{$\mathbb{Z}^+ = \{1, 2, 3, ...\}$. Also, $2 \times 2 = 4$.}

\begin{solution}
	$2$ is the only element in $\mathbb{Z}^+$ that satisfies $\varphi(x) = [x \times x = 4]$.
\end{solution}

\vfill


\problem{}
Try to define 2 in the structure $\Bigl( \mathbb{Z} ~\big|~ \{4, \times \} \Bigr)$. \par
Why can't you do it?

\begin{solution}
	This isn't possible. We could try $\varphi(x) = [x \times x = 4]$, but this is satisfied by both $2$ and $-2$. \par
	We have no way to distinguish between negative and positive numbers.

	\begin{instructornote}
		Actually, it is. Bonus problem: how? \par
		Do this after understanding quantifiers.
	\end{instructornote}
\end{solution}

\vfill


\problem{}
What numbers are definable in the structure $\Bigl( \mathbb{R}^+_0 ~\big|~ \{1, 2, \div \} \Bigr)$?

\begin{solution}
	We can define powers of two, positive and negative.

	If you're clever, you can define many more: $\sqrt{2}, \sqrt[3]{2}, ...$.
\end{solution}

\vfill
\pagebreak