Set(unofficial): a set is unordered collection of objects.
P={prime_numbers}
(let P be the set of prime numbers)
usually use capital numbers to represent set
elements
: number of sets
Roster Method: list of elements in the set (informal, typically written when sets are small and finite)
example: A={0,1,2,3,3,3,3,1}
example: B={0,1,2,3,...}
(need explanation sentence)
Variable Proposition: a statement involving unquantified variables which takes a truth value for appropriate values of the variables.
example: X>=8
(X is now undefined, but if it is defined, it will take a truth value)
variable proposition is not a proposition
you can interpret that as a function that used to qualify the context (output is a boolean)
Set Builder Notation
Important Number Sets:
\mathbb{N}=\{0,1,2,3,...\}: natural numbers (assume without proof)
\mathbb{Z}=\{...,-2,-1,0,1,2,...\}: integers (assume without proof)
\mathbb{Z}^{+}=\{1,2,3,...\}: positive integer (and you have TODO:z-)
Integer Divisibility: for a, b \in \mathbb{Z}, we say a divides(be inversely divisible by) b denoted a|b, if and only if there exist an integer c\in\mathbb{Z} such that b=ac.
0|0
is correct. 0=0\times1 (for instance) so the divisibility relation 0|0
holds true.Even and Odd: n\in\mathbb{Z} is even
if 2|n, and is odd
if 2|(n-1).
all integers are either even or odd by induction
zero is even because it is divisible by 2
Rational Numbers: \mathbb{Q}
\mathbb{Q}=\{\frac{a}{b} | a\in\mathbb{Z} and b\in\mathbb{Z}^{+}\} or
\mathbb{Q}=\{\frac{a}{b} | a, b\in\mathbb{Z} and b\neq 0\}
assume without proof
Real Numbers: \mathbb{R}
:warning: integer exponentiation is not within group because there are negative exponents
Empty Set: \{\} or \emptyset
For n\in\mathbb{Z}, [n]
means "a set consisting of the first n natural number"
[4]=\{1, 2, 3, 4\}
:warning: [0]=\{\}
[n] is always infinite even if n\in \mathbb{Z}^+
S = \{X|X\notin X\}, then "Is S\in S"
S is a set that contains sets
certainly \emptyset, \mathbb{Z} \in S
revised set definition: A set
is any collection of objects from a special universal set
. (and universal set
is not a set, do not call it set of all set
)
(A\in\{X|X\in X\} = \{A|A\in A\})
(A\in\{X|[whatever]\} = \{A|[whatever]\})
:question: (variable preposition \{\}\in\{\{\}\} returns Yes, and \{\}\in\{\} returns No, so {} is not the same as {{}})
:warning: \in just mean the word in
in python, and \subseteq means contains
in python.
Subset: A is subset
of B (A\subseteq B) if and only if every element of A is also an element of B. (not subset is \nsubseteq)
Equal: A\subseteq B \text{ and } B\subseteq A
Proper Subset(\subset or \subsetneq): A\subseteq B and A\neq B
Power set: Let A be a set, \mathcal{P}(A) is the set of all subset of A. (including A itself)
\emptyset, A \subseteq \mathcal{P}(A).
\mathcal{P}(A) = {\emptyset, {a}, {b}, A}
:warning: \mathcal{P}(\emptyset) = \{\emptyset\}
:warning: \mathcal{P}(\mathcal{P}(\emptyset)) = \{\emptyset, \{\emptyset\}\}
:warning: \mathcal{P}(\mathcal{P}(\mathcal{P}(\emptyset))) = \{\emptyset, \{\emptyset\}, \{\{\emptyset\}\}, \{\emptyset, \{\emptyset\}\}\}
P(P(P({})))={{}, P((P({})))}}
:=
is assignment symbol
\mathcal{P} (A\in\{X|[whatever]\} = \{A|[whatever]\})
:warning: natural numbers include 0, but [1] starts with 1.
TODO: How can we denote a pair of numbers as coordinates in set notation? (see piazza)
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