Paul R. Halmos - Naive Set Theory - 1960

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Created: January 14, 2016 / Updated: December 12, 2016 / Status: in progress / 5 min read (~883 words)

  • (p1) Sets have elements or members
  • (p1) Sets may be elements of sets
  • (p2) If $A$ and $B$ are sets and if every element of $A$ is an element of $B$, $A$ is a subset of $B$ or $B$ includes $A$
    • $A \subset B = B \supset A$
  • (p2) This implies $A \subset A$
    • The set inclusion is reflexive
  • (p2) If $A$ and $B$ are sets such that $A \subset B$ and $A \not= B$, then $A$ is a proper subset or $B$ or $B$ properly includes $A$ (proper inclusion)
  • (p2) If $A$, $B$ and $C$ are sets such that $A \subset B$ and $B \subset C$, then $A \subset C$ (set inclusion is transitive)
  • (p2) If $A \subset B$ and $B \subset A$, then A and B have the same elements and therefore, by the axiom of extension, $A = B$. (antisymmetric)
  • (p2) To prove that two sets are equals, you prove that $A \subset B$ and then $B \subset A$.
  • (p2) Inclusion is always reflexive and transitive
  • (p2) Belonging is never reflexive nor transitive
  • (p6) Nothing contains everything
  • (p7) There is no universe
  • (p8) There exists a set
    • (p8) There exists a set with no elements
    • Symbol: $\varnothing$ (empty set)
    • $\varnothing \subset A$ for every $A$
  • (p8) To prove that something is true (about the empty set), prove that it cannot be false
  • (p10) If $a$ is a set, we may form the unordered pair $\{a, a\}$. That unordered pair is denoted by $\{a\}$ and is called the singleton of $a$.
  • (p10) $a \in A \iff \{a\} \subset A$
  • (p13) $\bigcup\{X: X \in \varnothing\} = \varnothing$
  • (p13) $\bigcup\{X: X \in \{A\}\} = A$
  • (p13) $\bigcup\{X: X \in \{A, B\}\} = A \cup B$
  • (p13) $A \cup B = \{x: x \in A\ or\ x \in B\}$
  • (p14) $A \cap B = \{x \in A: x \in B\} = \{x \in B: x \in A\} = \{x: x \in A\ and\ x \in B\}$
  • (p15) If $A \cap B = \varnothing$, the sets $A$ and $B$ are called disjoint
  • (p15) $A \cap (B \cup C) = (A \cap B) \cup (A \cap C)$
  • (p15) $A \cup (B \cap C) = (A \cup B) \cap (A \cup C)$
  • (p17) $A - B = \{x \in A: x \not\in B\}$
  • (p17) De Morgan laws: $(A \cup B)' = A' \cap B'$, $(A \cap B)' = A' \cup B'$
  • (p18) $A + B = (A - B) \cup (B - A)$
  • (p20) The power set of a finite set with $n$ elements has $2^n$ elements.
  • (p22) Order c b d a = {c} {c, b} {c, b, d} {c, b, d, a}
  • (p23) Ordered pair (a, b) = {{a}, {a, b}}
  • (p24) Cartesian product: $A \times B = \{x: x = (a, b)\ for\ some\ a\ in\ A\ and\ some\ b\ in\ B\}$
  • (p27) Relation: $z = (x, y)$, $x R y$
  • (p27) Domain: $dom R = \{x: for\some\ y\ (x R y)\}$
  • (p27) Range: $ran R = \{y: for\some\ x\ (x R y)\}$
  • (p27) reflexive if $x R x$ for every $x$ in $X$
  • (p27) symmetric if $x R y$ implies $y R x$
  • (p27) transitive if $x R y$ and $y R z$ imply $x R z$
  • (p28) equivalence relation if it is reflexive, symmetric and transitive
  • (p28) A partition of $X$ is a disjoint collection $\mathcal{C}$ of non-empty subsets of $X$ whose union is $X$
  • (p45) A family $\{x_i\}$ whose index set is either a natural number or else the set of all naturals numbers is called a sequence
  • (p 47) No natural number is a subset of any of its elements
  • (p 47) Every element of a natural number is a subset of it
  • (p 48) Recursion theorem: If $a$ is an element of a set $X$, and if $f$ is a function from $X$ into $X$, then there exists a function $u$ from $\omega$ into $X$ such that $u(0) = a$ and such that $u(n^+) = f(u(n))$ for all $n$ in $\omega$.

Two sets are equal if and only if they have the same elements.

To every set $A$ and to every condition $S(x)$ there corresponds a set $B$ whose elements are exactly those elements $x$ of $A$ for which $S(x)$ holds.

For any two sets there exists a set that they both belong to.

For every collection of sets there exists a set that contains all the elements that belong to at least one set of the given collection.

For each set there exists a collection of sets that contains among its elements all the subsets of the given set.

There exists a set containing 0 and containing the successor of each of its elements.

  1. $0 \in \omega$
  2. $if\ n \in \omega,\ then\ n^+ \in \omega$
  3. $if\ S \subset \omega, if\ 0 \in \omega,\ and\ if\ n^+ \in S\ whenever\ n \in S,\ then\ S = \omega$
  4. $n^+\ \not= 0\ for\ all\ n\ in\ \omega$
  5. $if\ n\ and\ m\ are\ in\ \omega,\ and\ if\ n^+ = m^+,\ then\ n = m$

  • $\wedge$: and
  • $\vee$: or (either or both)
  • $\neg$: not
  • $\Rightarrow$: if-then (implies)
  • $\iff$: if and only if
  • $\exists$: for some (there exists)
  • $\forall$: for all

  • Halmos, Paul Richard. Naive set theory. Springer Science & Business Media, 1960.