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ADVENTURES IN THE VERIFICATION OF MATHEMATICS
by
Harvey M. Friedman
Computer Science Colloquium
Ohio State University
June 8, 2006
Verifying programs and verifying mathematics are two
distinct enterprises which are becoming more and more
closely related. Programs are to be verified by verifying
associated mathematical statements.
Mathematical statements arising from program verification
are believed to be much easier to deal with than statements
coming from serious mathematics. At least this is true for
“normal programming”.
I am optimistic about adjusting existing tools for math
verification to work wonders for program verification. This
depends on work by computer scientists on the design of
programming systems which automatically generate the simple
mathematical statements to be verified.
1.
2.
3.
4.
5.
6. Coming down to reality.
Why verify mathematics?
Proof assistants (generalities).
Proof assistants (more).
Some particular proof assistants.
Real algebra without distributivity. 1. COMING DOWN TO REALITY.
In the early days (1960’s) there was the idea that
computers could replace mathematicians, and prove serious
mathematical theorems entirely on their own.
Even to this day, the success along these lines is extremely limited, and this idea has been all but abandoned. An
exception is plane geometry, where beautiful things of
interest to humans are done solely by computers. See, e.g.,
S.C. Chou. Mechanical geometry theorem proving. Reidel,
Dortrecht, 1988.
W.T. Wu. Mechanical Theorem Proving in Geometries. Number
1 in Texts and Monographs in Symbolic Computation.
Springer, Wien, 1994. 2
But this is still very far from general purpose ordinary
mathematics, and worthless for program verification.
So why has this dream of unaided computer mathematics
collapsed?
The original idea of Hilbert was that there should be a
decision procedure for all of mathematics. This was refuted
by Turing, Gödel, Church, et al, in very strong ways.
E.g., there is no decision procedure for deciding whether a
sentence involving
" integers, ÿ, Ÿ, +, •
is true. This was later improved greatly (starting in 1970)
by the result that there is no decision procedure for
deciding whether a sentence
($x1,...,x9 Œ N)(P(x1,...,xn) = 0)
is true, where P is a polynomial with integer coefficients.
However, this is not even close to the full story.
Pointing to possible victory for computers was a number of
impressive decision procedures in very serious mathematical
contexts. These were known well before the 1960’s.
One is for all sentences involving
" integers, $ integers, ÿ, Ÿ, ⁄, Æ, ´, +, , <, 0, 1.
Another is for all sentences involving
" reals, $ reals, ÿ, Ÿ, ⁄, Æ, ´, +, , •, <, 0, 1.
Counterintuitive!, since reals are more sophisticated than
the integers, and we are getting away with •.
On the other hand, computational complexity considerations
enter in as follows.
The (Z,+,,0,1,<) decision problem is known to be
nondeterministic double exponential time complete. So all
known algorithms run in triple exponential time.
The (¬,+,,•,0,1,<) decision procedure is known to be 3
nondeterministic exponential time hard and exponential
space easy. Therefore, all known algorithms run in double
exponential time.
These are bad news, but the computers can fight back.
First of all, the decision procedure for (Z,+,,0,1,<)
works pretty well on real world examples, thanks to some
optimization work. It works splendidly on the universal
sentences in (Z,+,,0,1,<).
There is an exponential time decision procedure for the
universal sentences in (¬,+,,•,0,1,<), which is much
better than for the full theory.
The bad news is that there are all sorts of real world
examples of universal sentences in (¬,+,,•,0,1,<) where
the existing decision procedures blow up.
Exponential time algorithms themselves may or may not be
practical. The most well known ones in computer science are
those used to recognize satisfiability in propositional
calculus.
Any practical satisfiability recognizer can also
practically provide a truth assignment for any satisfiable
formula.
The propositional satisfiability problems that naturally
arise in the verification of mathematics are completely
clobbered by existing algorithms.
One can also make inroads into restricted sentences in
first order predicate calculus, with and without equality.
But satisfiability of sentences with even just a few (two?)
universal quantifiers in a binary function symbol, is not
decidable.
2. WHY VERIFY MATHEMATICS?
The verification of mathematics, in any reasonably general
purpose sense, now goes under the buzzword of
proof assistants
I.e., this is a human/machine interactive process. 4
But why do this at all? Some good reasons.
i. There is a subject called proof theory, in mathematical
logic. But it doesn’t say much about the structure of
actual proofs. We need to get our hands on actual
mathematical proofs in standardized form. There is the
belief that sophisticated proofs, which are much longer
than toy proofs, have features that do not appear in toy
proofs. This data and experience can only be conveniently
obtained through proof assistants.
ii. To make good on a philosophical claim made in the
foundations of mathematics. That there is an objective
standard for whether or not something has really been
proved. Justification of the special feature of mathematics
– certainty.
iii. To refute (conscious and unconscious) skeptics among
mathematicians, who, in some form, deny that mathematics is
capable of formalization. Whereas there may be senses in
which they are right, because of work on verification of
mathematics, we know that there are clear senses in which
they are wrong.
iv. To settle disputes as to whether or not something has
really been proved. This occurs infrequently, but has
occurred in connection with Kepler’s conjecture about
sphere packing. Also with the classification of finite
simple groups, there is unlikely to be any full record left
from living mathematicians for the future, without formal
verification.
v. To support the formal verification of software, and
computer systems in general.
3. Proof assistants (generalities).
Proof assistants are now very advanced in some respects,
with thousands of man years in them, including lots of
stuff thrown away. This has been going on since the 1960’s.
Pat Suppes was a very early pioneer at Stanford.
By now, as we shall see later, very serious mathematical
theorems continue to be formally verified through these
proof assistants. The mathematician sits interactively with
the proof assistant. The process is driven by the human,
who tries to get the proof assistant to accept the human’s 5
moves.
When successful, most proof assistants generate what is
called a “proof object”, which is a file containing a proof
in a very low level system. The file can be checked by an
independent program of a simple sort – incomparably simpler
than the code for the proof assistant itself.
Of course, then there is the question of how to verify this
simpler code. Verifying this simpler code in the original
assistant seems unsatisfactory. So a question is: in what
precise sense can we achieve certainty or near certainty?
Not clear.
1. The user orchestrates the refining of goals and
hypotheses according to a natural deduction framework. This
is in accordance with the general logical organization of
actual mathematical proofs.
2. The user cites definitions and theorems from
‘libraries’. The proper construction of libraries is
absolutely crucial in practice. Supports strong
reusability.
3. It is also crucial that the proof assistant be able to
make relatively trivial inferences on its own. Experience
shows that otherwise the process is just too time
consuming.
A lot of effort has gone into 1,2,3.
1) has stabilized long ago, although there is certainly a
lot of room for improvement in terms of readability of
output and user interfacing.
Readability of output has not been a high priority for
proof assistants, and is only recently being seriously
addressed.
3) uses a hodge podge of goodies that have been developed
over decades. These include
a. General purpose. Various general purpose simplification
procedures for expressions. These can be user directed, at
least in Isabelle. User can say what simplifies to what.
This is used to avoid having to enter simplified forms, and
also internally when the proof assistant tries to fill in
steps. 6
b. General purpose. Decision procedure for propositional
calculus. Various decision procedures for fragments of
predicate calculus with and without equality.
c. General purpose. Resolution theorem proving methods for
predicate calculus. This is an old method due to J.A.
Robinson, 1965. Since then it has been steadily improved,
and the one I hear most often about is called Otter.
d. Special purpose. Domain specific decision procedures for
various fragments of mathematics.
4. PROOF ASSISTANTS (MORE).
I am now going to go into some more detail about proof
assistants. I have taken some material from
Little Engines of Proof, by Natarjan Shankar, FME 2002:
Formal Methods – Getting it Right, Copengahen.
Available at
http://citeseer.ist.psu.edu/shankar02little.html
At the most general level, there have emerged two
approaches. One is represented by Alan Robinson's general
purpose resolution method, based on simple uniform
procedures guided by heuristics.
The other school pioneered by Hao Wang, pushes problem
specific combinations of decision and semidecision
procedures.
Current thinking is: abandon the first for the second. This
also incorporates the first as just one tool.
(Shankar) State of the art:
i. high powered propositional satisfiability solvers.
ii. ground decision procedures for equality and arithmetic.
iii. decision procedures for integers and reals, and
iv. abstraction methods for nicely approximating problems
over infinite domains.
There are also nice ways of combining different decision
procedures over different domains (with serious
limitations).
Not many relevant problems are stated in a form that is
readily attackable with existing decision procedures. 7
However, humans can decompose these problems into decidable
subproblems. Also to some extent, computers can too.
According to Shankar:
“The construction of modular inference procedures is a
challenging research issue in automated reasoning.
Work on little engines of proof has been gathering steam
lately. Many groups are actively engaged in the
construction of little proof engines, while others are
putting in place the train tracks on which these engines
can run.
PVS itself can be seen as an attempt to unify many
different inference procedures: typechecking, ground
decision procedures, simplification, rewriting, MONA
[EKM98], model checking [CGP99], abstraction, and static
analysis, within a single system with an expressive
language for writing mathematics.”
Along with SAT, two decision procedures stand out.
One is Presburger arithmetic, which in its full blown form
is the theory of (¬,Q,Z,<,0, 1,+,). This has a good
decision procedure that works pretty well in practice.
Another that we haven’t mentioned yet is WS1S = weak
monadic secondorder logic with 1 successor. The domains
are N and the collection of finite subsets of N, and we
have the successor function on N. From this we can easily
define <. This has a decision procedure that is
indefinitely iterated exponential time complete in theory,
OUCH!!, but is reasonably nice in practice.
In pure set theory, there is are a lot of implemented
decision procedures coming out of the SETL group led by
Jack Schwartz.
According to Shankar,
“WS1S is a natural formalism for many applications,
particularly for parametric systems. The logic can be used
to capture interesting datatypes such as regular
expressions, lists, queues, and arrays.”
Since so few natural problems fall within a single one of
these procedures, there has been considerable investigation 8
into how to combine different decision procedures.
The most well known and implemented method is the NelsonOppen procedure.
THEOREM (NelsonOppen). Suppose T1,...,Tn have disjoint
languages (except for =), and no finite models. Suppose the
universal fragments of each Ti are decidable. Then the
universal fragment of T1 » ... » Tn is decidable.
The procedure for the universal fragment of T1 » ... » Tn
may be very impractical, even though the procedures for the
Ti individually are practical. Works much better if the
theories have additional properties that are often met in
existing decision procedures.
The big limitation for this method is the hypothesis of
disjoint languages. Because of this, it turns out that,
generally, T1 » ... » Tn is too weak a theory to be all that
useful.
The point of recent joint work with Avigad is to study a
fundamental case of T1 » T2 where the languages are not
disjoint. We still obtain decidability of the universal
consequences (with difficultty), and some undecidability
results for more complicated consequences (also with
difficulty).
Shankar lists some challenges.
“The Complexity Challenge. Many decision procedures are of
exponential, superexponential, or nonelementary complexity.
However, this complexity often does not manifest itself on
practical examples. ... The challenge here is to understand
the ways in which one can overcome complexity bounds on
the problems that arise in practice through heuristic or
algorithmic means.
“The Theory Challenge. Inference procedures are hard to
build, extend, and maintain. The past experience has been
that good theory leads to simpler decision procedures with
greater efficiency. ... Methods derived by specializing
generalpurpose methods like resolution and rewriting can
also simplify the construction of decision procedures.
“The Modularity Challenge. As we have already noted,
inference procedures need rich programmer interfaces (APIs)
[BM86,FORS01]. Boyer and Moore [BM86] write: . . . the 9
black box nature of the decision procedure is frequently
destroyed by the need to integrate it. The integration
forces into the theorem prover much knowledge of the inner
workings of the procedure and forces into the procedure
many features that are unnecessary when the problem is
considered in isolation.
“The modularity challenge is a significant one. Butler
Lampson has argued that software components have always
failed at low levels of granularity (see
http://research.microsoft.com/lampson/Slides/ReusableCompon
entsAbstract.htm). He says that successful software
components are those at the level of a database, a
compiler, or a theorem prover, but not decision procedures,
constraint solvers, or unification procedures. For interoperation between inference components, we also need
compatible logics, languages, and term and proof
representations.
“The Integration Challenge. The availability of good
inference components is a prerequisite for integration, but
we also need to find effective ways of combining
these components in complementary ways. The combination of
decision procedures with model checking in predicate and
data abstraction is a case where such a complementary
integration is remarkably effective. Other such examples
include the combination of unification/matching procedures
and constraint solving, and typechecking and ground
decision procedures.
“The Verification Challenge. How do we know that our
inference procedures are sound? This question is often
asked by those who wish to apply inference procedures in
contexts where a high level of manifest assurance is
required. This question has been addressed in a number of
ways. ... Proof objects have also been widely used as a way
of validating inference procedures and securing mobile code
[Nec97]. ...
The verification of decision procedures is actually well
within the realm of feasible, and recently, there have been
several successful attempts in this direction
[Thie98,FS02].
5. SOME PARTICULAR PROOF ASSISTANTS.
Slides:
Formalization of Mathematics
Freek Wiedijk 10
Radboud University Nijmegen
TYPES Summer School 2005
GÄoteborg, Sweden
2005 08 23, 11:10
Freek lists four “prehistorical” proof assistants for
mathematics:
1968 Automath
Netherlands, de Bruijn
1971 nqthm US, Boyer & Moore
1972 LCF
UK, Milner
1973 Mizar
Poland, Trybulec
Freek lists seven current systems for mathematics
Mizar ......most mathematical
LCF‡HOL‡Isabelle... most pure
Automath‡ Coq..most logical
NuPRL
PVS.........most popular
nqthm ‡ ACL2.....most
computational
Arrows also point from Mizar to Isabelle, and from LCF to
Coq and PVS. Also an arrow from nqthm to PVS.
From Freek:
Formalizing 100 Theorems
Theorems not formalized yet in italics.
The Irrationality of the Square Root of 2
Fundamental Theorem of Algebra
The Denumerability of the Rational Numbers
Pythagorean Theorem
Prime Number Theorem 11
Gödel's Incompleteness Theorem
Law of Quadratic Reciprocity
The Impossibility of Trisecting the Angle and Doubling the
Cube
The Area of a Circle
Euler's Generalization of Fermat's Little Theorem
The Infinitude of Primes
The Independence of the Parallel Postulate
Polyhedron Formula
Euler's Summation of 1 + (1/2)^2 + (1/3)^2 + ....
Fundamental Theorem of Integral Calculus
Insolvability of General Higher Degree Equations
De Moivre's Theorem
Liouville's Theorem and the Construction of Trancendental
Numbers
Four Squares Theorem
All Primes (1 mod 4) Equal the Sum of Two Squares
Green's Theorem
The NonDenumerability of the Continuum
Formula for Pythagorean Triples
The Undecidability of the Continuum Hypothesis
SchroederBernstein Theorem
Leibnitz's Series for Pi
Sum of the Angles of a Triangle
Pascal's Hexagon Theorem 12 Feuerbach's Theorem
The Ballot Problem
Ramsey's Theorem
The Four Color Problem
Fermat's Last Theorem
Divergence of the Harmonic Series
Taylor's Theorem
Brouwer Fixed Point Theorem
The Solution of a Cubic
Arithmetic Mean/Geometric Mean
Solutions to Pell's Equation
Minkowski's Fundamental Theorem
Puiseux's Theorem
Sum of the Reciprocals of the Triangular Numbers
The Isoperimetric Theorem
The Binomial Theorem
The Partition Theorem
The Solution of the General Quartic Equation
The Central Limit Theorem
Dirichlet's Theorem
The CayleyHamilton Thoerem
The Number of Platonic Solids
Wilson's Theorem 13
The Number of Subsets of a Set
Pi is Trancendental
Konigsberg Bridges Problem
Product of Segments of Chords
The HermiteLindemann
Transcendence Theorem
Heron's Formula
Formula for the Number of Combinations
The Laws of Large Numbers
Bezout's Theorem
Theorem of Ceva
Fair Games Theorem
Cantor's Theorem
L'Hôpital's Rule
Isosceles Triangle Theorem
Sum of a Geometric Series
e is Transcendental
Sum of an arithmetic series
Greatest Common Divisor Algorithm
The Perfect Number Theorem
Order of a Subgroup
Sylow's Theorem
Ascending or Descending Sequences
The Principle of Mathematical Induction 14
The Mean Value Theorem
Fourier Series
Sum of kth powers
The CauchySchwarz Inequality
The Intermediate Value Theorem
The Fundamental Theorem of Arithmetic
Divergence of the Prime Reciprocal Series
Dissection of Cubes (J.E. Littlewood's "elegant" proof)
The Friendship Theorem
Morley's Theorem
Divisibility by 3 Rule
Lebesgue Measure and Integration
Desargues's Theorem
Derangements Formula
The Factor and Remainder Theorems
Stirling's Formula
The Triangle Inequality
Pick's Theorem
The Birthday Problem
The Law of Cosines
Ptolemy's Theorem
Principle of Inclusion/Exclusion
Cramer's Rule
Bertrand's Postulate 15 Buffon Needle Problem
Additions from Freek:
AtiyahSinger Index Theorem
Baker's Theorem on Linear Forms in Logarithms
BlackScholes Formula
BorsukUlam Theorem
Cauchy's Integral Theorem
Cauchy's Residue Theorem
Chen's theorem
Classification of Finite Simple Groups
Gödel's Completeness Theorem
Gödel's Second Incompleteness Theorem
GreenTao Theorem
Fundamental Theorem of Galois Theory
HeineBorel Theorem
Hilbert Basis Theorem
Hilbert Nullstellensatz
HilbertWaring theorem
Invariance of Dimension
Jordan Curve Theorem
Lie's work relating Algebras and Groups
Nash's Theorem
Perelman's proof of the Poincaré Conjecture
Stoke's Theorem
StoneWeierstrass Theorem
Thales' Theorem
Yoneda lemma
(last modification 20060523)
State of the art: recent big formalizations.
PRIME NUMBER THEOREM
Jeremy Avigad e.a.
1 megabyte = 30,000 lines = 42 files of Isabelle/HOL
the elementary proof by Selberg from 1948
FOUR COLOR THEOREM
Georges Gonthier:
(2.5 megabytes = 60,000 lines = 132 files of Coq 7.3.1
streamlined proof by Robertson, Sanders, Seymour & Thomas
from 1996
Contains very interesting `own' proof language on top of
Coq heavily relies on reflection `this formalization really 16
needs Coq'
JORDAN CURVE THEOREM
Tom Hales:
2.1 megabytes = 75,000 lines = 15 files of HOL Light.
Proof thru the Kuratowski characterization ofplanarity.
Current Biggies:
Formalization of a complete `advanced' mathematics
textbook:
A Compendium of Continuous Lattices, by Gierz et al.
Project led by Grzegorz Bancerek
about 70% formalized
4.4 megabytes = 127,000 lines = 58 files of Mizar.
Flyspeck project.
Kepler, 1661:
Is the way we customarily stacks oranges the most efficient
way to stack spheres?
Tom Hales, 1998: yes !
Proof depends on computer checking 3 gigabytes programs &
data, couple of months of computer time. FlysPecK project:
`Formal Proof of Kepler'
So why did the qed project not take off as intended?
Reason ONE: incompatible systems. set theory type theory
higher order logic PRA classical constructive
extensional intensional
impredicative predicative
choice only countable choice
There are projects underway to try to put systems together.
Michael Kohlhase, and Carsten Schurmann.
Reason TWO: why mathematicians are not interested (yet)
the cost is too high. . .
de Bruijn factor =
size of formalization ÷
size of normal text
question: is this a constant? 17
experimental: around 4
de Bruijn factor in time =
time to formalize ÷
time to understand
much larger than 4.
formalizing one textbook page = 1 man/week = 40 man hours
. . . and the gain is too little
NOT impossibly expensive
formalizing all of undergraduate mathematics = 140 man
years the price of about one Hollywood movie.
BUT: after formalization we just have a big
incomprehensible file. We don't have a good argument yet
for spending that money.
AND: it does not look like mathematics. Even in Mizar and
Isar: still looks like code.
Mizar is based on set theory but it is a typed system.
Mizar types are soft types:
M : N(t1; : : : ; tn)
should really be read as a predicate.
Think of Mizar types as predicates that the system keeps
track of for you.
Mizar Mathematical Library
the biggest library of formalized mathematics
49,588 lemmas
1,820,879 lines of `code'
64 megabytes
165 `authors'
912 `articles'
Mizar versus Isar
some reasons to prefer Mizar over Isar
the set theory of Mizar is much more powerful and
expressive than the HOL logic of Isabelle/HOL 18
Mizar is much more able to talk about abstract mathematics,
and in particular about algebraic structures, with nice
notation
dependent types are way cool
some reasons to prefer Isar over Mizar
Isabelle gives you an interactive system
Isabelle allows you to mix declarative and procedural proof
Isabelle has much more possibilities of automation
Isabelle allows you to define binders
Mizar is about as complex as the Pascal programming
language
Will proof assistants ever become common among
mathematicians?
The experienced user's answer: 50 years.
6. Real algebra without distributivity.
Decision procedures for the reals with both addition and
multiplication exist (Tarski) but are quite bad in
practice. There are probably a number of reasons for this.
One particular reason is that it is very hard to automate
the judgment of whether or not distributivity should be
applied. r(s+t) = rs + rt. Sometimes yes, sometimes no.
So one idea is to have the user always control use of
distributivity, and let the computer try to handle
everything else.
This leads to fragments of the usual theory of the reals,
where distributivity is dropped.
One of the theories that we study is written T[Q] = Tadd[Q]
» Tmult[Q], where
Tadd[Q] is based on the symbols
0,1,+,,<,fa, a Œ Q 19 and Tmult[Q] is based on the symbols
0,1,•,÷,<,fa, a Œ Q
Here fa is scalar multiplication by the rational Q.
Tadd[Q] and Tmult[Q] consist of the true sentences in their
respective languages. They have very elegant complete
axiomatizations.
It is not at all clear just what T[Q] proves or doesn’t.
Even the purely universal sentences.
THEOREM 1. There is a decision procedure for determining
whether a universal sentence in the language of T[Q] is
provable in T[Q].
It is not clear whether this can be made efficient. However
THEOREM 2. Theorem 1 for equations can be done efficiently.
THEOREM 3. If Hilbert’s 10th problem fails for Q (believed)
then there is no decision procedure for determining whether
an existential sentence in the language of T[Q] provable in
T[Q].
THEOREM 4. There is no decision procedure for determining
whether a """$...$ sentence in the language of T[Q] is
provable in T[Q].
REFERENCES
Formalization of mathematics, Freek Wiediuk, Radboud
University Nijmegen, TYPES Summer School 2005, GÄoteborg,
Sweden, 2005 08 23, 11:10, lecture notes.
Little Engines of Proof, by Natarjan Shankar, FME 2002:
Formal Methods – Getting it Right, Copengahen,
http://citeseer.ist.psu.edu/shankar02little.html
J. Avigad, and H. Friedman, Combining decision procedures
for the reals, January 31, 2006, submitted,
http://www.math.ohiostate.edu/%7Efriedman/ ...
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This note was uploaded on 08/05/2011 for the course MATH 366 taught by Professor Joshua during the Fall '08 term at Ohio State.
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 Math, The Land

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