Download Mechanical Theorem Proving________________ The Intellectual

Mechanical Theorem
Proving____
The Intellectual Excitement of Computer Science
Group Members
Elita Cheung
Lily Irani
Paul Tenney
Introduction
Mechanical theorem proving is an
important subject in artificial intelligence
Even though Turing showed that there is
no general decision procedure to check
the validity of formulas of the first-order
logic, there are proof procedures which
can verify that a formula is valid if indeed
it is valid...
Our Research Journey
• Journals about automated theorem
proving
• Difficult and technical material required
background we lacked
• Talked with professors, read about basic
logic
Overview of Automated
Theorem Proving
• Philosophical issues regarding a
mechanical theorem prover
• Theory and history of the field -- lesson
in logic
• Applications of automated theorem
provers
Quick History and Theory
• Principles of Automated Theorem Proving
heavily based on symbolic logic
• Learning the basic vocabulary and
concepts was essential to understanding
those principles
• The history of this field can be easier
understood along with theories
• Quick lesson in symbolic logic J
Higher Order
First Order
Propositional
More interactive
More Expressive
Different sorts of logic...
Propositional Logic
• A proposition is a declarative sentence
that is either true or false (it cannot be
both).
• Examples of propositions: ”Stuff at
Stanford Shopping Mall is expensive",
”Elita is a bargain hunter", ”Elita is shopaholic at Stanford mall".
Propositional Logic
• B Stuff at Stanford Shopping Mall is expensive
C Elita is a bargain hunter
D Elita is a shop-aholic at Stanford Mall
• Symbols, such as B, C, D, that are used to
denote propositions are called atoms
Simple symbols...
Not

Or

And

If… then

If and only if

Propositional Logic
• Example: The sentence "If stuff at
Stanford Shopping mall is expensive and
Elita is a bargain hunter, then Elita is not
a shop-aholic at Stanford Mall" can be
represented by
(( B C) (D))
• As we see, this compound proposition
can represent a complicated idea that we
deal with in everyday life.
Propositional Logic
• Truth Table
G H G (G H) (G  H) (GH) (GH)
T T
F
T
T
T
T
T F
F
F
T
F
F
F T T
F
T
T
F
F F
F
F
T
T
T
Propositional Logic
• The assignment of truth values {T,F} to
{G, H} is one of four interpretations of
formula F  (G  H)
• Equivalent formulas
• Example: Suppose that bike accidents increase
if there are more freshmen on campus. Also,
suppose that students will start building their own
impact airbags for their bikes when bike accidents
increase. Assume that there are more freshmen
on campus. Show that you can conclude that
students will starting building their own airbags.
Propositional Logic Example...
The four following statements correspond
to this example:
1. If there are more freshmen on campus,
the bike accidents increase
2. If bike accidents increase, students
start building bike airbags
3. More freshmen on campus
4. Students will start building bike airbags
First Order Logic
• First order logic is a more expressive
logic than propositional logic. For
example, propositional logic cannot
denote the following:
P: Every man is mortal
Q: Confucius is a man
R: Confucius is mortal
First Order Logic
• First order logic has three more logical
notions than propositional logic
• terms, predicates, and quantifiers
• Most of mathematical and everyday
language can be symbolized by the firstorder logic.
First Order Logic - New Terms
• Predicate
• Quantifier
• Interpretation -- different from propositional
• "An interpretation of a formula F in the first-order logic consists of a
nonempty domain D, and an assignment of 'values' to each constant,
function symbol, and predicate symbol occurring in F as follows:
– To each constant, we assign an element in D.
– To each n-place function symbol, we assign a mapping
from D^n to D.
– To each n-place predicate symbol, we assign a mapping
from D^n to {T, F}."
First Order Logic - New Terms
• Satisfiable- A formula P is satisfiable
(consistent) if and only if there exists an
interpretation I such that P has a truth value
of True in I.
• Unsatisfiable
Herbrand’s theorem… and a
little history
• Leibniz (1646-1716) tried to prove
validity of formula
• Turing and Church (1936)
• Herbrand’s contribution
• Robinson’s Resolution
Resolution
• Herbrand’s procedure’s problem: amount
of time needed to implement increase
exponentially (too many interpretations
to generate!)
• Resolution decreases the number of
interpretations
Resolution
• The basic idea of the resolution principle
is to check rather any set S of clauses
contains the empty clause •
. If S
contains •
, then S is unsatisfiable. If S
does not contain •
, then check to see if •
can be derived from S. If it can, then it is
also unsatisfiable.
• Example in propositional logic
• Example in first order logic
Propositional Resolution
• For propositional logic, the principle can
be roughly described as the following:
combine the literal that are
complementary to each other so that they
cancel out (e.g. P and ~P are
complementary).
• Example in propositional logic
First Order Resolution
• substitution and unification
• Example in first order logic
First Order Resolution
• S = {T(x,y,u,v) v P(x,y,u,v), P(x,y,u,v) v
E(x,y,v,u,v,y), T(a,b,c,d), E(a,b,d,c,d,b)}
1. T(x,y,u,v) v P(x,y,u,v)
2. P(x,y,u,v) v E(x,y,v,u,v,y)
3. T(a,b,c,d)
4. E(a,b,d,c,d,b)
5. ~P(a,b,c,d)
a resolvent of 2 and 4
6.~T(a,b,c,d)
a resolvent of a and 5
7. •
a resolvent of 3 and 6
Applied Theory
• First order specifications
• Boyer and Moore’s Induction
Intel Pentium Chip
Specification - IEEE level 74
• “when rounding towards negative infinity,
the result shall be the format’s value ...
closest to and no greater than the
infinitely precise result”
Informal
Intel Pentium Chip
Specification - IEEE level 74
round(toNegInf, R, V) =
(R <= V) ^ (V < R + ulp+)
R = result, V = value to be rounded,
ulp+ = smallest representable increment
Formal (First Order)
Induction
Algorithm
Applications
• Mathematical proof checking
• The QED Project
• Computer chip verifications
• Software verification
Mathematical Proof Checking
• Automated theorem
provers do not
“automate” math
• “Debugs” proofs
• Hard to use many
proof checkers
The QED Project
• Effort of scientists from many laboratories and
institutions
• Will represent
mathematical
knowledge, technique
• Based on a few pages
of math
• Still in early stages
“The development of mathematics
towards a greater appreciation has
led... to the formalization of
large tracts of it, so that one can
prove any theorem using nothing
but a few mechanical rules.”
-K.Gödel
The QED Project- Hoped
Benefits
• Reduce mathematical “noise pollution.”
• Speed publication of papers by taking focus
off of proof checking. Referees can focus on
relevance.
• Cultural monument to mathematics.
Chip Verification
• Formal vs.
testbench
• Comparison
verification
• NP-Complete
• IBM, Intel, AMD
successes
Software Verification
• Hardware is more economically viable
• More design effort put into software
• => Software verification is viable
• Especially useful for critical applications:
safety, e-commerce, military
Software Verification Paradox
• What will verify the
verification program?
• Pragmatism does not
demand ideal accuracy
• Significant improvement
enough
More Information
Our website:
• demonstrations of
theorem proving tools
online
• additional research
Credits
Thank you to Professor David Dill for
information and support through e-mail
and in person.