Deduction and Induction Elementary deduction, my dear Watson…
Truth, deduction, computation lecture i (last one)
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Truth, Deduction, ComputationLecture ISet Theory, ZFC axioms
Vlad PatryshevSCU2013
Sets, Formally (ZFC)
Ernst Friedrich Ferdinand
Zermelo (pronounced [tsermelo])
Abraham Fraenkel
Axiom of Choice
ZFC Axioms
1. Extensionality Axiom2. Axiom of Empty Set3. Comprehension Axiom Schema (filter)4. Axiom of Pair5. Axiom of Union6. Replacement Axiom Schema (map)7. Axiom of Infinity8. Powerset Axiom9. Foundation Axiom
10. Axiom of Choice
Extensionality
That’s it. We can prove symmetry, associativity, reflexivity.
A=B ≡ ∀x ((x∈A) ↔ (x∈B))
Axiom of Empty Set
Why do we need it?
Because all other axioms assume existence of sets; this one states an existence.
∃A ∀x (¬x∈A)denoted as ∅
Comprehension Axiom Schema
● Note that we have no rule allowing to build something like {x | x=x} or even {x | x=1}
● Do you see that {x∈A | true} = A?● It does not give us ∅● Also, it does not give us union of two sets
A is a set ⊢ {x∈A | P(x)} is a set
Axiom of Pairing
● Notation: {a,b} = {x∈?|x=a v x=b}
(here we made a choice of an A… not very pure)
● x∈{a,b} ↔ (x=a v x=b)● Now we can, given an x, build {x}● We can also build an unlimited number of sets: ∅,
{∅}, {{∅}}, {{{∅}}}...● Introduce ordered pair: {x, {x,y}} (problems?)● Kuratowski pair: {{x}, {x,y}}
∀x ∀y ∃A (x∈A ∧ y∈A)
Axiom of Union
∪{y∈A} ≡ {x∈? | ∃y (y∈A)}
X ∪ Y ≡ ∪{a∈{X,Y}} = {x∈? | x∈X v x∈Y}
E.g.● A={{a,b}, {b,c,d}} ∪A = {a,b,c,d} ● ∪∅ = ∅● ∪{x} = x
∀A ∃B ∀y ∀x((x∈y ∧ y∈A) → x∈B)
Replacement Axiom Schema
In other words, if we have a function f defined on A, we can say that its image B is a set.
∀A (∀x(x∈A → (∃!y P(x,y))) ⊢ ∃B ∀x(x∈A → (∃y (P(x,y)∧ y∈B))
Axiom of Infinity
Do you recognize Peano Natural Numbers?The axiom says: we have natural numbers.
∃X (∅∈X ∧ ∀y (y∈X → S(y)∈X))where S(y) ≡ y ∪ {y}
Axiom of Powerset
P(X) ≡ {z∈?/*maybe an y from above*/|z⊂X}
● A⊂B here means non-strict subset. E.g. A⊂A● Now we have ℵ
1● Can |x| = |P(x)|? No (see Russell paradox)
∀x∃y∀z (z⊂x → z∈y)where A⊂B ≡ ∀x ((x∈A)→(x∈B))
Well-founded Relationship
● xRy means (x,y) ∈ R● E.g. partial order on N, a>b● Counterexample: a<b on N● Counterexample: aRb, bRc, cRa● Can use generalized induction (Noether)
R ⊂ A×A is well-founded iff∀S⊂A (S=∅ v (∃x∈S ∀y∈S ¬xRy))
Foundation Axiom
● No set is an element of itself (Quine atoms outlawed)● No infinite descending sequence of sets exists● No need for Kuratowski pairs● For every two sets, only one can be an element of the
other
∀x (x=∅ v (∃y∈x (¬∃z(z∈y∧z∈x))
Axiom of Choice (AC)
What it means● every set can be fully ordered● f:X→Y is surjection ⊢ f has an inverse● Banach-Tarski paradox
Alternatively: Axiom of Determinacy
∀x (∅∉x v (∃f:(x→∪x) (∀y∈x f(y)∈y))
There’s a Java implementation
https://gist.github.com/vpatryshev/7711870
That’s it. Thanks for listening.