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Set Theory Basics

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Subset

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A set where all its elements are contained within another set. Notation: ABA \subseteq B.

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Union of sets

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The set of all distinct elements that are in either set. Notation: ABA \cup B.

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Set difference

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The set of elements that are in one set but not in the other. Notation: ABA - B or ABA \setminus B.

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Symmetric difference

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The set of elements which are in either of the sets and not in their intersection. Notation: ABA \triangle B.

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Codomain of a function

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The set that contains all possible outputs of a function. Notation: If f:ABf: A \to B, then BB is the codomain.

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One-to-one function (Injective)

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A function where each element of the range is mapped to by exactly one element of the domain. Notation: ff is injective if f(a)=f(b)f(a) = f(b) implies a=ba = b.

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Cardinality of a set

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The number of elements in a set. Notation: A|A| or #A\#A.

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Empty set

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A set with no elements. Notation: \emptyset or {}\{\}.

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Complement of a set

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The set of all elements in the universal set that are not in the given set. Notation: AcA^c or Aˉ\bar{A}.

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Partition of a set

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A division of a set into non-overlapping subsets that cover all elements of the original set.

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Universal set

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The set that contains all objects or elements under consideration. Notation: Usually denoted by UU or ξ\xi.

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Disjoint sets

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Two sets with no common elements. Notation: AB=A \cap B = \emptyset.

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Equivalence relation

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A relation that is reflexive, symmetric, and transitive. Notation: aba \sim b indicates aa is equivalent to bb.

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Well-defined function

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A function where the output value is uniquely determined by the input, adhering to the definition of a function.

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Ordered pair

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A pair of elements with an order, where the first element is considered the first component of the pair. Notation: (a,b)(a, b).

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Domain of a function

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The set of all possible inputs for which the function is defined. Notation: If f:ABf: A \to B, then AA is the domain.

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Power set

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The set of all subsets of a set, including the empty set and the set itself. Notation: P(A)\mathcal{P}(A) or 2A2^A.

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Cartesian product of sets

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The set of all ordered pairs obtained by the product of two sets. Notation: A×BA \times B.

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Function (mapping)

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A relation between sets that associates each element of a set with exactly one element of another set. Notation: f:ABf: A \to B.

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Bijection (Bijective function)

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A function that is both injective and surjective, meaning it is a one-to-one correspondence between the domain and codomain. Notation: ff is bijective if it is both one-to-one and onto.

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Inverse of a function

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A function that reverses the direction of a given function, if such a function exists. Notation: f1f^{-1} for the inverse of ff.

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Onto function (Surjective)

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A function where each element of the codomain is the image of at least one element of the domain. Notation: ff is surjective if for every yy in the codomain, there is an xx in the domain such that f(x)=yf(x) = y.

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Intersection of sets

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The set of elements that are common to both sets. Notation: ABA \cap B.

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Proper subset

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A subset that is not equal to the parent set. Notation: ABA \subsetneq B.

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Image of a function

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The set of all outputs that the function actually produces. Notation: If f:ABf: A \to B and AA' is a subset of AA, then f(A)f(A') is the image of AA'.

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