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Lecture Outline

Forms of Introductory Logic Propositional (or Sentential) Logic. Lecture 2: Propositional Logic Wffs, and the view. Valuation of a formula, tables Satisfiability, Alan Mycroft Effectiveness, Feasibility. Compactness . Computer Laboratory, University of Cambridge, UK http://www.cl.cam.ac.uk/˜am21

MPhil in ACS – 2011/12

Alan Mycroft (University of Cambridge) Introductory Logic – Lecture 2 MPhil in ACS – 2011/12 1 / 1 Alan Mycroft (University of Cambridge) Introductory Logic – Lecture 2 MPhil in ACS – 2011/12 2 / 1

Forms of Logic Syntax 1

We assume a (countable) = A , A ,... of propositional A { 1 2 } There are many formulation of logic; all model what we think of as variables. The logical connectives are , , , , . {∧ ∨ ¬ → ↔} logic, but some are more sophisticated than others: The set of well-formed formulae (wffs), ranged over by σ, is the A smallest set such that: Propositional (or Sentential) Logic. No variables. Predicate Logic. Adds variables, , . Role of equality. A ⊆ A ∀ ∃ whenever σ then ( σ) Modal Logic. Adds a notion of modality – e.g. temporal in ∈ A ¬ ∈ A whenever σ, σ then (σ σ ) which we can talk about time and express statements like “once A 0 ∈ A ∧ 0 ∈ A has become true then it remains true”. ditto for , , . ∨ → ↔ Intuitionistic Logic. Disallows reasoning based on such as Note that this is an inductive definition of set . A “A A” (justification: Gödel and others). ∨ ¬ Formally all wffs are fully bracketed, but we elide them for humans: We start with the simplest form. e.g. A B C. ¬ ∨ ∧ [‘ ’ binds tightest, then ‘ ’, then ‘ ’.] ¬ ∧ ∨

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Syntax 2

How do we determine when a wff is true, or false? For propositional variables we need a truth assignment (or valuation) v : B to tell us. A−→ Computer Scientists have an additional to specify Don’t confuse ‘ ’ ( space) and ‘ ’ (implication in the logic). −→ → inductively-defined sets like that of wffs – we write BNF: We extend v to all wffs (not just propositional variables) with a function σ ::= A σ σ0 σ σ0 σ σ σ0 σ σ0 v : B using truth tables: | ∧ | ∨ | ¬ | → | ↔ A−→ Seen as a grammar on strings this is ambiguous, but seen as a v(A) = v(A) grammar on trees then this is fine. v(σ σ0) = true if v(σ) = true and v(σ ) = true ∧ 0 = false otherwise

v(σ σ0) = see over ∨

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Semantics 2 Semantics 3

What we’re really doing is modelling ‘real’ ‘and’, ‘or’ etc. in the logic. v(A) = v(A) Indeed, if we write AND : B B B (and similarly for the other v(σ σ0) = true if v(σ) = true and v(σ ) = true × −→ ∧ 0 connectives) then the equations simply mirror our informal = false otherwise understanding of logic within the formal logic:

v(σ σ0) = true if v(σ) = true or v(σ0) = true ∨ v(A) = v(A) = false otherwise v(σ σ0) = AND(v(σ), v(σ0)) v( σ) = true if v(σ) = false ∧ ¬ v(σ σ0) = OR(v(σ), v(σ0)) = true otherwise ∨ v( σ) = NOT (v(σ)) v(σ σ0) = true if v(σ) = false or v(σ0) = true ¬ → v(σ σ0) = IMP(v(σ), v(σ0)) = false otherwise → v(σ σ0) = EQV (v(σ), v(σ0)) v(σ σ0) = true if v(σ) = v(σ ) ↔ ↔ 0 = false otherwise

Alan Mycroft (University of Cambridge) Introductory Logic – Lecture 2 MPhil in ACS – 2011/12 7 / 1 Alan Mycroft (University of Cambridge) Introductory Logic – Lecture 2 MPhil in ACS – 2011/12 8 / 1 Truth tables Truth tables 2

People who have done Computer Hardware/Digital Electronics have The functions AND, OR etc. can be written as truth tables: seen this all before.

AND true false OR true false NOT true One particular aspect is that any function B B B can be × · · · × −→ true true false true true true true false written as a composition of AND, OR and NOT . false false false false true false false true We say AND, OR, NOT is universal for boolean functions. { } IMP true false EQV true false true true false true true false Note that NAND is also universal where { } false true true false false true NAND(x, y) = NOT (AND(x, y))

as is NOR . { }

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Satisfaction, Tautology Tautologous Implication

A particularly interesting question is when does wff σ ‘imply’ wff σ0. We could write σ σ but we’re interested in situations where the LH → 0 σ behaves like a and the RH σ0 behaves like a question (or some hypotheses and a conclusion). We say, given a wff σ that: valuation v satisfies σ if v(σ) = true So we write σ = τ where = is part of our mathematics, not part of the | | σ is satisfiable if there is a valuation which satisfies σ logic. σ is a tautology if every valuation satisfies σ In fact we generalise to the form Σ = τ where Σ is a set of wffs and | define Σ = τ to hold σ is unsatisfiable if no valuation satisfies σ | if whenever a valuation satisfies all σ Σ then it also satisfies τ ∈

[Note the use of ‘hold’ when we’re talking about maths (the meta-level) rather than ‘is true’ which is a value within the logic.]

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Tautology revisited Computability, Feasibility

Given wff σ consider = σ, often written = σ. ∅ | | By vacuous reasoning this holds whenever σ is a tautology. Determining whether a (in propositional logic) is Some tautologies: satisfiable or a tautology is (effectively) computable, as indeed A A (excluded middle) almost anything else about propositional logic. ∨ ¬ (A B) A B (de Morgan) But satisfiablity is NP-complete, often called infeasible as the ¬ ∨ ↔ ¬ ∧ ¬ (A B) A B (de Morgan) best-known algorithm is exponential in the number of variables in ¬ ∧ ↔ ¬ ∨ ¬ the worst case. A (B C) (A B) (A C) (distributivity) ∧ ∨ ↔ ∧ ∨ ∧ However there’s a growth industry in SAT solvers which get fast A (B C) (A B) (A C) (distributivity) ∨ ∧ ↔ ∨ ∧ ∨ results on many cases which arise in practice.

Meta-theorem (duality): swapping and in a tautology only involving ∧ ∨ , , , gives another tautology. ∧ ∨ ¬ ↔

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Digression: Truth versus Compactness

One more tricky question concerns the behaviour of Σ = τ when Σ is | Intuitively there are two ways to show two expressions are equivalent: infinite (perhaps not even countable). show they have the same output value for every input value Note the the more members we put in Σ the less satisfiable it becomes do algebraic manipulations on both until they are syntactically (and vice versa): equal. A is satisfiable { } Note that (interpreting ‘’ as ‘wff’) we have only exploited the A is satisfiable {¬ } former version here – which is easy and computable because there A, A is not satisfiable { ¬ } are only a finite number of possible input values. We have seen that sometimes odd things happen when we move to infinite sets, so the question we ask is (compactness): So we haven’t bothered with the latter (which corresponds to proof). Is the behaviour of Σ = τ explained by the behaviour of But it will rise in prominence when we turn to predicate calculus (a.k.a. | Σ = τ where Σ ranges over all finite of Σ. first-order logic) when the range of input values may be infinite. 0 | 0 We answer the question in the affirmative.

Alan Mycroft (University of Cambridge) Introductory Logic – Lecture 2 MPhil in ACS – 2011/12 15 / 1 Alan Mycroft (University of Cambridge) Introductory Logic – Lecture 2 MPhil in ACS – 2011/12 16 / 1 Compactness Theorem

Notation: a set Σ of wffs is satisfiable if there is a truth assignment which satisfies every σ Σ. ∈ Theorem (compactness): a set Σ of wffs is satisfiable iff every finite Σ Σ is satisfiable. 0 ⊆ Equivalent form of compactness: if Σ = τ then there is a finite Σ Σ | 0 ⊆ such that Σ = τ. 0 | Why is this important? Reading Σ as a set of hypotheses (allowed to be infinite) which imply τ, we want to be able to construct a textual proof (which must be finite and so can’t use an infinite number of hypotheses in Σ).

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