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Archive-name: sci-math-faq/fermat
Last-modified: February 20, 1998
Version: 7.5
Fermat's Last Theorem
History of Fermat's Last Theorem
Pierre de Fermat (1601-1665) was a lawyer and amateur mathematician.
In about 1637, he annotated his copy (now lost) of Bachet's
translation of Diophantus' Arithmetika with the following statement:
Cubum autem in duos cubos, aut quadratoquadratum in duos
quadratoquadratos, et generaliter nullam in infinitum ultra
quadratum potestatem in duos ejusdem nominis fas est dividere:
cujus rei demonstrationem mirabilem sane detexi. Hanc marginis
exiguitas non caperet.
In English, and using modern terminology, the paragraph above reads
as:
There are no positive integers such that x^n + y^n = z^n for n>2.
I've found a remarkable proof of this fact, but there is not enough
space in the margin [of the book] to write it.
Fermat never published a proof of this statement. It became to be
known as Fermat's Last Theorem (FLT) not because it was his last piece
of work, but because it is the last remaining statement in the
post-humous list of Fermat's works that needed to be proven or
independently verified. All others have either been shown to be true
or disproven long ago.
What is the current status of FLT?
Theorem 1 [Fermat's Last Theorem] There are no positive integers x, y,
z, and n > 2 such that x^n + y^n = z^n.
Andrew Wiles, a researcher at Princeton, claims to have found a proof.
The proof was presented in Cambridge, UK during a three day seminar to
an audience which included some of the leading experts in the field.
The proof was found to be wanting. In summer 1994, Prof. Wiles
acknowledged that a gap existed. On October 25th, 1994, Prof. Andrew
Wiles released two preprints, Modular elliptic curves and Fermat's
Last Theorem, by Andrew Wiles, and Ring theoretic properties of
certain Hecke algebras, by Richard Taylor and Andrew Wiles. The first
one (long) announces a proof of, among other things, Fermat's Last
Theorem, relying on the second one (short) for one crucial step.
The argument described by Wiles in his Cambridge lectures had a
serious gap, namely the construction of an Euler system. After trying
unsuccessfully to repair that construction, Wiles went back to a
different approach he had tried earlier but abandoned in favor of the
Euler system idea. He was able to complete his proof, under the
hypothesis that certain Hecke algebras are local complete
intersections. This and the rest of the ideas described in Wiles'
Cambridge lectures are written up in the first manuscript. Jointly,
Taylor and Wiles establish the necessary property of the Hecke
algebras in the second paper.
The new approach turns out to be significantly simpler and shorter
than the original one, because of the removal of the Euler system. (In
fact, after seeing these manuscripts Faltings has apparently come up
with a further significant simplification of that part of the
argument.)
The papers were published in the May 1995 issue of Annals of
Mathematics. For single copies of the issues send e-mail to
jlorder@jhunix.hcf.jhu.edu for further directions.
In summary:
Both manuscripts have been published. Thousands of people have a read
them. About a hundred understand it very well. Faltings has simplified
the argument already. Diamond has generalized it. People can read it.
The immensely complicated geometry has mostly been replaced by simpler
algebra. The proof is now generally accepted. There was a gap in this
second proof as well, but it has been filled since October 1994.
Related Conjectures
A related conjecture from Euler
x^n + y^n + z^n = c^n has no solution if n is >= 4
Noam Elkies gave a counterexample, namely 2682440^4 + 15365639^4 +
18796760^4 = 20615673^4. Subsequently, Roger Frye found the absolutely
smallest solution by (more or less) brute force: it is 95800^4 +
217519^4 + 414560^4 = 422481^4. "Several years", Math. Comp. 51 (1988)
825-835.
This synopsis is quite brief. A full survey would run too many pages.
References
[1] J.P.Butler, R.E.Crandall,& R.W.Sompolski, Irregular Primes to One
Million. Math. Comp., 59 (October 1992) pp. 717-722.
Fermat's Last Theorem, A Genetic Introduction to Algebraic Number
Theory. H.M. Edwards. Springer Verlag, New York, 1977.
Thirteen Lectures on Fermat's Last Theorem. P. Ribenboim. Springer
Verlag, New York, 1979.
Number Theory Related to Fermat's Last Theorem. Neal Koblitz, editor.
Birkhduser Boston, Inc., 1982, ISBN 3-7643-3104-6
Did Fermat prove this theorem?
No he did not. Fermat claimed to have found a proof of the theorem at
an early stage in his career. Much later he spent time and effort
proving the cases n=4 and n=5. Had he had a proof to his theorem
earlier, there would have been no need for him to study specific
cases.
Fermat may have had one of the following ``proofs'' in mind when he
wrote his famous comment.
* Fermat discovered and applied the method of infinite descent,
which, in particular can be used to prove FLT for n=4. This method
can actually be used to prove a stronger statement than FLT for
n=4, viz, x^4 + y^4 = z^2 has no non-trivial integer solutions. It
is possible and even likely that he had an incorrect proof of FLT
using this method when he wrote the famous ``theorem''.
* He had a wrong proof in mind. The following proof, proposed first
by Lame' was thought to be correct, until Liouville pointed out
the flaw, and by Kummer which latter became and expert in the
field. It is based on the incorrect assumption that prime
decomposition is unique in all domains.
The incorrect proof goes something like this:
We only need to consider prime exponents (this is true). So
consider x^p + y^p = z^p. Let r be a primitive p-th root of unity
(complex number)
Then the equation is the same as:
(x + y)(x + ry)(x + r^2y)...(x + r^(p - 1)y) = z^p
Now consider the ring of the form:
a_1 + a_2 r + a_3 r^2 + ... + a_(p - 1) r^(p - 1)
where each a_i is an integer
Now if this ring is a unique factorization ring (UFR), then it is
true that each of the above factors is relatively prime.
From this it can be proven that each factor is a pth power from
which FLT follows. This is usually done by considering two cases,
the first where p divides none of x, y, z; the second where p
divides some of x, y, z. For the first case, if x + yr = u*t^p,
where u is a unit in Z[r] and t is in Z[r], it follows that x = y
(mod p). Writing the original equation as x^p + (-z)^p = (-y)^p,
it follows in a similar fashion that x = -z (mod p). Thus 2*x^p =
x^p + y^p = z^p = -x^p (mod p) which implies 3*x^p = 0 (modp) and
from there p divides one of x or 3|x. But p>3 and p does not
divides x; contradiction. The second case is harder.
The problem is that the above ring is not an UFR in general.
Another argument for the belief that Fermat had no proof ---and,
furthermore, that he knew that he had no proof--- is that the only
place he ever mentioned the result was in that marginal comment in
Bachet's Diophantus. If he really thought he had a proof, he would
have announced the result publicly, or challenged some English
mathematician to prove it. It is likely that he found the flaw in his
own proof before he had a chance to announce the result, and never
bothered to erase the marginal comment because it never occurred to
him that anyone would see it there.
Some other famous mathematicians have speculated on this question.
Andre Weil, writes:
Only on one ill-fated occasion did Fermat ever mention a curve of
higher genus x^n + y^n = z^n, and then hardly remains any doubt
that this was due to some misapprehension on his part [...] for a
brief moment perhaps [...] he must have deluded himself into
thinking he had the principle of a general proof.
Winfried Scharlau and Hans Opolka report:
Whether Fermat knew a proof or not has been the subject of many
speculations. The truth seems obvious ... [Fermat's marginal note]
was made at the time of his first letters concerning number theory
[1637]... as far as we know he never repeated his general remark,
but repeatedly made the statement for the cases n=3 and 4 and posed
these cases as problems to his correspondents [...] he formulated
the case n=3 in a letter to Carcavi in 1659 [...] All these facts
indicate that Fermat quickly became aware of the incompleteness of
the [general] ``proof" of 1637. Of course, there was no reason for
a public retraction of his privately made conjecture.
However it is important to keep in mind that Fermat's ``proof"
predates the Publish or Perish period of scientific research in which
we are still living.
References
From Fermat to Minkowski: lectures on the theory of numbers and its
historical development. Winfried Scharlau, Hans Opolka. New York,
Springer, 1985.
Basic Number Theory. Andre Weil. Berlin, Springer, 1967
_________________________________________________________________
--
Alex Lopez-Ortiz alopez-o@unb.ca
http://www.cs.unb.ca/~alopez-o Assistant Professor
Faculty of Computer Science University of New Brunswick
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