• Mathematics without infinitely small, continuous mathematical objects. The mathematics of finite sets.

Propositional calculus

  • Comes from the linguistic concept that things can be either true or false.

  • We should avoid variables when forming statements, as they may change the logical value.

    • \(2=7\) statement
    • \(x=5\) not a statement

  • In logic we do not use the equals sign, we use the equivalence sign \(\equiv\).

  • Logical values (booleans) are denoted by either 0 or 1 (or t, f, etc.).

  • When doing logic, we use propositional variables (e.g. p, q, r).

    • Can be either true or false.

  • The operations done on propositional variables are called propositional connectives.

    • Conjunction: \(p \land q\) is only true if both p and q are true \((0001)\)
    • Disjunction: \(p \lor q\) is only false if both p and q are false \((0111)\)
    • Implication (material conditional): \(p \implies q\) is false only if p is true and q is false (truth table \((1011)\))
      • \(\equiv \neg p \lor q\)

  • Not necessarily connectives but unary operations:

    • Negation: Denoted by ~, \(\neg\) or NOT, negates the one input \((10)\).

  • A (propositional) formula is a "properly constructed" logical expression.

    • e.g. \(\neg[(p \lor q)] \land r\)
    • \((p \land)\) is not a formula, as \(\land\) requires 2 variables.
    • Logical equivalence: \(\phi(p, q, k) \equiv \psi(p, q, k)\), logical value of \(\phi\) is equal to logical value of \(\psi\).
    • Commutativity: \(p \land q \equiv q \land p\)
    • Associativity: \((p \land q) \land r \equiv p \land (q \land r)\)
    • Distributivity: \(p \land (q \lor r) \equiv (p \land q) \lor (p \land r)\)
    • Conjunctive normal form: every formula can be written as a conjunction of one or more disjunctions.
      • \(\neg(B \lor C)\) can be written as \(\neg B \land \neg C\)

  • Double negation law: \(\neg(\neg p) \equiv p\)

  • De Morgan's laws: \(\neg(p \land q) \equiv \neg p \lor \neg q\) and \(\neg(p \lor q) \equiv \neg p \land \neg q\).

  • If and only if (iff): \(p \iff p \equiv (p \implies q) \land (q \implies p)\)

  • Contraposition law:

    • \((p \implies q) \equiv (\neg q \implies \neg p)\) prove by contraposition

      • \((p \implies q) \equiv (\neg p \lor q)\)
      • \((\neg q \implies \neg p) \equiv (\neg (\neg q) \lor (\neg p) \equiv (q \lor \neg p) \equiv (\neg p \lor q)\)

    • Contradiction law:

      • \(p \lor \neg p \equiv 1\) and \(p \land \neg p \equiv 0\)

  • Tautology: \(\phi (p, q, ... r)\) is a tautology iff \(\phi \equiv 1\)

Sets

  • We will consider subsets of universal set \(\mathbb X\)

    • \(2^\mathbb X = \{ A : A \subseteq \mathbb X\}\)
    • \(2^\mathbb X = P(\mathbb X)\)
    • All 2 object subsets of \(\mathbb X\): \(P_2(\mathbb X)\)

  • \(A \subset B \equiv\) every element of A is an element of B \(\equiv \{x \in \mathbb X : x \in A \implies x \in B\}\)

  • Operations on sets:

    • Union - \(\cup\) - \(A \cup B = \{ x \in \mathbb X : x \in A \lor x \in B \}\)
    • Intersection - \(\cap\) - \(A \cap B = \{ x \in \mathbb X : x \in A \land x \in B \}\)
    • Complement - \(A'\) - \(A' = \{ x \in \mathbb X : \neg (x \in A) \}\)
      • If \(x = \{ 1 \}\) then \(x' = \emptyset\)

  • Equality of sets: \(A = B\) iff \(x \in \mathbb X : (x \in A \iff x \in B)\)

  • Difference of sets:

    • \(A \setminus B = \{ x \in \mathbb X : x \in A \land x \notin B \} = A \cap B'\)
    • Symmetric difference: \(A \div B = (A \setminus B) \cup (B \setminus A)\)

  • Laws of set algebra:

    • \(A \cup B = B \cup A , A \cap B = B \cap A\)
    • \((A \cup B) \cup C = A \cup (B \cup C), (A \cap B) \cap C = A \cap (B \cap C)\)
    • \((A \cap (B \cup C) = (A \cap B) \cup (A \cap C)\) vice versa
    • \(A \cap \emptyset, A \cap \mathbb X = A, A \cup \emptyset = A, A \cup \mathbb X = \mathbb X\)
    • \((A \cup B)' = A' \cap B'\) vice versa
    • \(A \cup A' = \mathbb X, A \cap A' = \emptyset\)

  • Note: \(\{ \emptyset \} \neq \emptyset\), one is a set with one element, one is the empty set, no elements (\(\{ \}\))

  • Quip: \(\{ x \in \mathbb R : x^2 = -1\} = \emptyset\)

Quantifiers

  • \(\phi\) - prepositional function: yields only true or false value
  • \(\forall\) means "for all" and \(\exists\) means "there exists"

  • \[\forall\]

    • Shorthand for \[\land\] e.g. \[(\forall x \in \{ 1, 2, ... 10 \}) x > 0 \equiv 1 > 0 \land 2 > 0 \land ... 10 > 0\]

  • \[\exists\]

    • Shorthand for \[\lor\] e.g. \[(\exists x \in \{ 1, 2, ... 10 \}) x > 5 \equiv 1 > 5 \lor 2 > 5 \lor ... 10 > 5\]

  • \(\neg \forall \equiv \exists\), vice versa

  • With quantifiers we can write logical statements e.g.

    • \((\forall x \in \mathbb{R}) (\forall y \in \mathbb{R}) x > y\) is a statement and is false
    • \((\forall x) (\exists y) x > y\) is true
    • shortcut: \((\exists x, y) \equiv (\exists x) (\exists y)\)

  • Quantifiers can be expressed in set language, sort of a definition in terms of sets:

    • \((\forall x \in \mathbb{X}) (\phi(x)) \equiv \{ p \in \mathbb{X} : \phi(p) \} = \mathbb{X}\)
    • \((\exists x \in \mathbb{X}) (\phi(x)) \equiv \{ q \in \mathbb{X} : \phi(q) \} \neq \emptyset\)
    • \((\exists x \in \mathbb{X}) (\neg \phi(x)) \equiv \neg ( \{ p \in \mathbb{X} : \phi(p) \} = \mathbb{X} )\)

  • Order of quantifiers matters.

Relations

  • Cartesian product:

    • \(A \times B = \{ (p, q) : p \in A \land q \in B \}\)

  • Def: A relation \(R\) on a set \(\mathbb X\) is a subset of \(\mathbb X \times \mathbb X\) (\(R \subseteq \mathbb X \times \mathbb X\))

  • Graph of a function \(f()\): \(\{ (x, f(x) : x \in Dom(f) \}\)

  • Properties of:

    1. Reflexivity: \((\forall x \in \mathbb X ) (x, x) \in R \equiv (\forall x \in \mathbb X) x R x\)
    2. Symmetricity: \([ (\forall x, y \in \mathbb X) (x, y) \in R \implies (y, x) \in R) ] \equiv [ (\forall x, y \in \mathbb X) ( x R y \implies y R x) ]\)
    3. Transitivity: \((\forall x, y, z \in \mathbb X) (x R y \land y R z \implies x R z)\)
    4. Antisymmetricity: \((\forall x, y \in \mathbb X) (x R y \land y R x \implies x = y)\)

  • Equivalence relations:

    • Def: \(R \subseteq \mathbb X \times \mathbb X\) is said to be an equivalence relation iff \(R\) is reflexive, symmetric and transitive.
    • Congruence modulo n: \(p R q \equiv n | p - q\)
    • Def R - and equivalence relation of \(\mathbb X\): The equivalence class of an element \(x \in \mathbb X\) is the set \([x]_R = \{ y \in \mathbb X : x R y \}\)
      • Every \(x \in \mathbb X\) belongs to the equivalence class of some element \(a\).
      • \((\forall x, y \in \mathbb X) ([x] \cap [y] \neq \emptyset \iff [x] = [y])\)

  • Partitions

    • A partition is a set containing subsets of some set \(\mathbb X\) such that their collective symmetric difference equals \(\mathbb X\). A partition of is a set \(\{ A_i: i \in \mathbb I \land A_i \subseteq \mathbb X \}\) such that:

      • \((\forall x \in \mathbb X) (\exists j \in \mathbb I) (x \in A_j)\)
      • \((\forall i, j \in \mathbb I) (i \neq j \implies A_i \cap A_j = \emptyset)\)

    • \(\{ A_i \}_{i \in \mathbb I}\) is a partition iff there exists an equivalence relation \(R\) on \(\mathbb X\) such that:

      • \((\forall i \in \mathbb I) (\exists x \in \mathbb X) A_i = [x]_R\)
      • \((\forall x \in \mathbb X) (\exists j \in \mathbb I) [x] = A_j\)

    • The quotient set: \(\mathbb X / R = \{ [a] : a \in \mathbb X \}\)