Prerequisites: logic

Set

A set is a collection of distinct objects, considered as an object in its own right. Sets are usually denoted by capital letters, e.g. .
The objects in a set are called the elements or members of the set. We write if is an element of the set , and if is not an element of .
denotes the empty set, which contains no elements.
B means that every element of is also an element of .

Cardinality

The cardinality of a set is the number of elements in .
We denote the cardinality of by , also denoted by or .

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Two sets and are equal if and only if both are subsets of each other.

Ways of denoting sets.

Roster notation: or listing elements (“aufzählend”)
Set builder notation: , where is a property that must satisfy (“beschreibend”), e.g.:
Subsets: e.g.: (more common)
Interval notation:

Union (Vereinigung)

The union of two sets and , denoted by , is the set of all elements that are in or in or in both:

Or for set systems (more than two sets):

is the index set (the set of indices for the sets which we are taking the union of).

Intersection ((Durch)Schnitt)

The intersection of two sets and , denoted by , is the set of all elements that are in both and :

Or for set systems (more than two sets):

Complement

Let’s call the set of all elements we are working with (Grundmenge / basic set). Then we can define the complement of a set as the set of all elements in that are not in :

Other notations for the complement:

Difference

The difference of two sets and , denoted by , is the set of all elements that are in but not in :

Symmetric difference

The symmetric difference of two sets and , denoted by , is the set of all elements that are in exactly one of the two sets (like an xor):

(NOTE: Due to a bug in Obsidian-tikzjax, the symmetric difference only renders correctly in live-preview mode… click the link for correct rendering)

Properties of sets

Proving set identities

“Rückführung auf logische Regeln” (reduction to logical rules):

“Anwenden der Äquivalenz” (applying the equivalence):

Element table: Create a table with and for each set, then compare the tables, just like a truth table in logic.

power set

Power set

The power set is the set of all subsets of , i.e.:

It can also be written as .
Why? Because it represents all possible binary choices for the elements in .
For example: has the subsets , which can be represented as in binary, i.e. subsets.

The cardinality of the power set of a finite set : .

$$

{ 0,1 }^{A} \mapsto 2^{A}

$\{0,1\}^A$ represents the set of all functions from set $A$ to the set $\{ 0,1 \}$. $2^{A}$ represents the [[power set]] – the set of all subsets of $A$ → For any subset of $A$, we can define a function $f: A \to \{ 0,1 \}$ that maps elements in the subset to $1$ and the rest to $0$. → Conversely, for any function $f: A \to \{ 0,1 \}$, we can define a subset of $A$ that contains all elements that are mapped to $1$. $\implies$ There is a [[bijective|bijection]] between $2^{A}$ and the set of all functions from $A$ to $\{ 0,1 \}$.

The sum of all binomial coefficients for fixed (row of Pascal’s triangle) equals

This makes intuitive sense when thinking about sets: counts the ways to choose items from items, and summing over all possible gives us the total number of possible subsets of an -element set, which is . Each element either is or isn’t in the subset, giving us two choices for each of the elements → power set.

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cartesian product

Cartesian product

The cartesian product of two sets and , denoted by , is the set of all possible ordered pairss where and :


e.g.: and , then

The cartesian product is not commutative:

Taking the cartesian product of mutiple sets:

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multiset

Multiset

A multiset is a modification of the concept of a set that allows for multiple instances of the same element. Unlike regular sets where each element appears exactly once, multisets track the multiplicity (number of occurrences) of each element.
Formally, a multiset over a universe (base set) is defined via its “multiplicity function”:

Key properties:

  • Order is irrelevant (like regular sets)
  • Elements can appear multiple times
  • The multiplicity of each element is significant
  • Elements can have zero occurrences

Notation: If is a multiset, we can denote it as:

  • (listing all occurrences)
  • (using superscripts for multiplicity)

The cardinality of a multiset includes all instances:

Examples

The string “hello” as a multiset:
Chemical formula H₂O as a multiset:

Permuting a multiset is like permuting a string of characters:
There are ways to permute a set with elements, but for a multiset, we need divide by the amount of permutations of equal elements whose order doesn’t matter (eliminating all those sub-trees). E.g. has permutations, but only unique ones.
Let be a multiset with elements where is an element and is the multiplicity of .
The number of unique permutations of is:

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A note on terminology

https://mathworld.wolfram.com/Collection.html says that a collection refers to a multiset.
But gpt5 says that’s almost certainly not the case in most contexts, a collection just a set.
But collection is so generic and context dependent, I only use it as “a bunch of things” and then define if necessary, else use a more specific term.