Sets: Summary

• inclusion relation is reflexive, transitive, and anti-symmetric

• proper inclusion relation is irreflexive, transitive, and asymmetric.

• inclusion and proper inclusion relations have natural converses;

  A (properly) includes B iff B is (properly) included in A;

  alternatively, A is a (proper) superset of B

• exclusion relation is symmetric and anti-reflexive

• subset is an example of inclusion relation

• inclusion relation is antisymmetric

• set membership (belonging) is an example of membership relation

• membership (elementhood) relation is reflexive

• inclusion relation is transitive, membership relation is not

Relations: Summary

(x,y) ∈ R or xRy (x is "R-related" to y)

R: X -> Y

A relation $R$ from set $X$ (domain) to set $Y$ (codomain), where $x \in X$ and $y \in Y$, is denoted as $(x,y) \in R$ or $xRy$ (x is R-related to y).

If the sets are equal, the Cartesian product, $X\times X$, is denoted by $X^2$ and a relation is $xRx$

A relation is a subset of the Cartesian product, $R \subseteq X\times Y$

A relation is an element of the powerset of the Cartesian product of sets, $R \in \mathcal{P}(X\times Y)$

Total number of relations of an n-element set with itself is $|\mathcal{P}(X^2)| = 2^{n^2}$

Sets

• Cardinality of the set X is denoted by $|X| = n$
• Cardinality of the powerset of the set X is $|\mathcal{P}(X)| = 2^{|X|} = 2^n$
• Cardinality of the Cartasian product of the set X: $|X \times X| = |X^2| = n^2$
• Cardinality of the powerset of the Cartasian product of the set X is $|\mathcal{P}(X^2)| = 2^{(n^2)}$
• Each relation is a subset of the Cartasian product of the set X: $R \subseteq X^2$
• Each relation is an element in the powerset of the Cartasian product of the set X: $R \in P(X^2)$
• Number of all possible relations on a $X^2$ set: $|\mathcal{P}(X^2)| = 2^{(n^2)}$
• standard set notation: curly braces for listing the elements explicitly

• comprehensions: {x ∈ S |...}

• ∅ for the empty set

• S \ T for the set difference of S and T

• cardinality of a set S is |S|

• powerset of S i.e. the set of all the subsets of S, is P(S)

• the set {0,1,2,3,4,5,...} of natural numbers is denoted by the symbol N

• a set is countable if its elements can be placed in one-to-one correspondence with the natural numbers

Relations

• An n-place relation on a collection of sets S1,S2,...,Sn

  is a set R ⊆ S1 × S2 ×...× Sn of tuples of elements S1 - Sn

• the elements s1 ∈ S1 through sn ∈ Sn are related by R if (s1,...,sn) is an element of R

• A one-place relation on a set S is called a predicate on S

• P is true of an element s ∈ S if s ∈ P

• we write P(s) instead of s ∈ P, regarding P as a function mapping elements of S to truth values

• A two-place relation R on sets S and T is a binary relation

• We write sRt instead of (s, t) ∈ R

• if S and T are the same set U, then R is a binary relation on U

• 3-place or more place relations are often written using a mixfix concrete syntax, where the elements in the relation are separated by a sequence of symbols that jointly constitute the name of the relation. For example, for the typing relation for the simply typed lambda calculus, we write Γ ⊢ s : T to mean the triple (Γ,s,T) is in the typing relation.

• The domain of a relation R on sets S and T, written dom(R), is the set of elements s ∈ S such that (s, t) ∈ R for some t.

• The codomain or range of R, written range(R), is the set of elements t ∈ T such that (s, t) ∈ R for some s

Functions

• A relation R on sets S and T is called a partial function from S to T if, whenever (s, t1) ∈ R and (s, t2) ∈ R, we have t1 = t2.

• If, in addition, dom(R) = S, then R is called a total function (or just function) from S to T.

• A partial function R from S to T is said to be defined on an argument s ∈ S if s ∈ dom(R), and undefined otherwise.

• We write f (x) ↑, or f (x) =↑, to mean f is undefined on x, and f (x)↓ to mean f is defined on x

• we also need to define functions that may fail on some inputs, so it is important to distinguish failure (which is a legitimate, observable result) from divergence

• a function that may fail can be either partial, i.e. it may also diverge, or total (it must always return a result or explicitly fail)

• We write f(x)=fail when f returns a failure result on the input x.

• Formally, a function from S to T that may also fail is actually a function

  from S to T ∪ {fail}, where we assume that fail does not belong to T.

• Suppose R is a binary relation on a set S and P is a predicate on S. P is preserved by R if whenever we have sRs' and P(s), we also have P(s0).