# set-interactions

## Set interactions

<https://en.wikipedia.org/wiki/Subset>

Two sets, A and B, may interact (or be related) in several ways:

## Disjoint sets

A and B do not interact, they are not related in any way; they are completely disjoint (non-overlapping) from one another.

![Disjoint sets](https://upload.wikimedia.org/wikipedia/commons/thumb/d/df/Disjunkte_Mengen.svg/320px-Disjunkte_Mengen.svg.png)

Sets $$x$$ and $$y$$ may interact or be related in several ways:

* $$x$$ is a member of $$y$$: $$x\in y$$ (membership relation)
* $$x$$ is a subset of $$y$$: $$x\subseteq y$$ (inclusion relation)
  * $$x$$ is a proper subset of $$y$$: $$x\subset y$$
  * $$x$$ is equal to $$y$$ if $$x\subseteq y \land y\subseteq x$$

Axioms:

* every set is a subset of itself, $$A\subseteq A$$
* the empty set is a subset of every set, $$\varnothing \subseteq A$$
* therefore, the empty set is a subset of itself, $$\varnothing \subseteq \varnothing$$

## Subset relation

Besides membership (which relates elements and sets), another fundamental relation is the **subset relation**, also called the **inclusion relation**, which is relation between sets, denoted by the symbol $$\subseteq$$.

If all elements of a set $$X$$ are also elements of a set $$Y$$, then $$X$$ is the **subset** of $$Y$$, denoted by

$$X \subseteq Y$$

At the same time, set $$Y$$ is a **superset** of $$X$$, denoted by $$Y \supseteq X$$

```
Given objects x and y
(y = {x}) → (∀x . x ∈ y ∧ [x ⊆ y → ∀z.z ∈ x ∧ z ∈ y])
  then x ∈ y holds always,
  but  x ⊆ y holds only if x is a set, e.g.:
    (x=1 → y={1}) → (x ∈ y ∧ x ⊈ y)
    if (x={1}) → (y={{1}}), then x ∈ y and x ⊆ y
```

* ∈ ∋ ∉ ∌
* ∣ ∅ ⋜ ⋝
* ∃ ∀ ∄
* ⋃ ⋂
* ⊆ ⊇ ⊂ ⊃&#x20;
* ⊊ ⊋ ⊄ ⊅
* ⊈ ⊉

$$
\text{Given } a,b\ where\ b = {a} \\
\text{if } a = 1\ \text{then } b = {1}\ ,so\ a \in b\\
\text{if } a = {1}\ \text{then } b = {{1}}\ ,so\ a\in b \land a \subseteq b\\
$$

A set $$X$$ is a **proper subset** of a set $$Y$$, denoted by $$X\subset Y$$ if set $$Y$$ has additional elements besides those that are also in set $$X$$.

That is, if every element of $$X$$ is an element of $$Y$$ and $$|X| < |Y|$$.

For example, $$X={1,2}$$ and $$Y={1,2,3,4}$$.

Here, $$X\subseteq Y$$, but, more precisely, $$X$$ is a proper subset of $$Y$$ i.e. $$X\subset Y$$.

> Every set is a subset of itself:\
> $${a,b,c} \subseteq {a,b,c}$$