# RB_Fermat.html

PIERRE DE FERMAT (1601 - 1665)

From A Short Account of the History of Mathematics' (4th edition,
1908) by W. W. Rouse Ball.

While Descartes was laying the foundations of analytical geometry, the
same subject was occupying the attention of another and not less
distinguished Frenchman. This was Fermat. Pierre de Fermat, who was
born near Montauban in 1601, and died at Castres on January 12, 1665,
was the son of a leather-merchant; he was educated at home; in 1631 he
obtained the post of councillor for the local parliament at Toulouse,
and he discharged the duties of the office with scrupulous accuracy
and fidelity. There, devoting most of his leisure to mathematics, he
spent the remainder of his life - a life which, but for a somewhat
acrimonious dispute with Descartes on the validity of certain analysis
used by the latter, was unruffled by any event which calls for special
notice. The dispute was chiefly due to the obscurity of Descartes, but
the tact and courtesy of Fermat brought it to a friendly conclusion.
Fermat was a good scholar, and amused himself by conjecturally
restoring the work of Apollonius on plane loci.

Except a few isolated papers, Fermat published nothing in his
lifetime, and gave no systematic exposition of his methods. Some of
the most striking of his results were found after his death on loose
sheets of paper or written in the margins of works which he had read
and annotated, and are unaccompanied by any proof. It is thus somewhat
difficult to estimate the dates and originality of his work. He was
constitutionally modest and retiring, and does not seem to have
intended his papers to be published. It is probable that he revised
his notes as occasion required, and that his published works represent
the final form of his researches, and therefore cannot be dated much
earlier than 1660. I shall consider separately (i) his investigations
in the theory of numbers; (ii) his use in geometry of analysis and of
infinitesimals; and (iii) his method for treating questions of
probability.

(i) The theory of numbers appears to have been the favourite study of
Fermat. He prepared an edition of Diophantus, and the notes and
comments thereon contain numerous theorems of considerable elegance.
Most of the proofs of Fermat are lost, and it is possible that some of
them were not rigorous - an induction by analogy and the intuition of
genius sufficing to lead him to correct results. The following
examples will illustrate these investigations.

(a) If p be a prime and a be prime to p then a^(p-1) - 1   is
divisible by p, that is, a^(p-1) - 1 \equiv 0   (mod p). A proof of
this, first given by Euler, is well known. A more general theorem is
that a^(\phi(n)) - 1 \equiv 0 (mod n)   (mod n), where a is prime to n
and \phi(n)   is the number of integers less than n and prime to it.

(b) An odd prime can be expressed as the difference of two square
integers in one and only one way. Fermat's proof is as follows. Let n
be the prime, and suppose it equal to x² - y², that is, to (x + y)(x -
y). Now, by hypothesis, the only integral factors of n are n and
unity, hence x + y = n and x - y = 1. Solving these equations we get x
= ½ (n + 1) and y = ½ (n - 1).

(c) He gave a proof of the statement made by Diophantus that the sum
of the squares of two integers cannot be of the form 4n - 1; and he
added a corollary which I take to mean that it is impossible that the
product of a square and a prime of the form 4n - 1 [even if multiplied
by a number prime to the latter], can be either a square or the sum of
two squares. For example, 44 is a multiple of 11 (which is of the form
4 × 3 - 1) by 4, hence it cannot be expressed as the sum of two
squares. He also stated that a number of the form a² + b², where a is
prime to b, cannot be divided by a prime of the form 4n - 1.

(d) Every prime of the form 4n + 1 is expressible, and that in one way
only, as the sum of two squares. This problem was first solved by
Euler, who shewed that a number of the form 2^m (4n + 1) can be always
expressed as the sum of two squares.

(e) If a, b, c, be integers, such that a² + b² = c², then ab cannot be
a square. Lagrange gave a solution of this.

(f) The determination of a number x such that x²n + 1 may be a square,
where n is a given integer which is not a square. Lagrange gave a
solution of this.

(g) There is only one integral solution of the equation x² + 2 = y³;
and there are only two integral solutions of the equation x² + 4 = y³.
The required solutions are evidently for the first equation x = 5, and
for the second equation x = 2 and x = 11. This question was issued as
a challenge to the English mathematicians Wallis and Digby.

(h) No integral values of x, y, z can be found to satisfy the equation
x^n + y^n = z^n ; if n be an integer greater than 2. This proposition
has acquired extraordinary celebrity from the fact that no general
demonstration of it has been given, but there is no reason to doubt
that it is true.

Probably Fermat discovered its truth first for the case n = 3, and
then for the case n = 4. His proof for the former of these cases is
lost, but that for the latter is extant, and a similar proof for the
case of n = 3 was given by Euler. These proofs depend on shewing that,
if three integral values of x, y, z can be found which satisfy the
equation, then it will be possible to find three other and smaller
integers which also satisfy it: in this way, finally, we shew that the
equation must be satisfied by three values which obviously do not
satisfy it. Thus no integral solution is possible. It would seem that
this method is inapplicable to any cases except those of n = 3 and n =
4.

Fermat's discovery of the general theorem was made later. A proof can
be given on the assumption that a number can be resolved into the
product of powers of primes in one and only one way. The assumption
has been made by some writers; it is true of real numbers, but it is
not necessarily true of every complex number. It is possible that
Fermat made some erroneous supposition, but, on the whole, it seems
more likely that he discovered a rigorous demonstration.

In 1823 Legendre obtained a proof for the case of n = 5; in 1832
Lejeune Dirichlet gave one for n = 14, and in 1840 Lamé and Lebesgue
gave proofs for n = 7. The proposition appears to be true universally,
and in 1849 Kummer, by means of ideal primes, proved it to be so for
all numbers except those (if any) which satisfy three conditions. It
is not certain whether any number can be found to satisfy these
conditions, but there is no number less than 100 which does so. The
proof is complicated and difficult, and there can be no doubt is based
on considerations unknown to Fermat. I may add that, to prove the
truth of the proposition, when n is greater than 4 obviously it is
sufficient to confine ourselves to cases when n is a prime, and the
first step in Kummer's demonstration is to shew that one of the
numbers x, y, z must be divisible by n.

The following extracts, from a letter now in the university library at
Leyden, will give an idea of Fermat's methods; the letter is undated,
but it would appear that, at the time Fermat wrote it, he had proved
the proposition (h) above only for the case when n = 3.

Je ne m'en servis au commencement qe pour demontrer les propositions
negatives, comme par exemple, qu'il n'y a aucu nombre moindre de
l'unité qu'un multiple de 3 qui soit composé d'un quarré et du
triple d'un autre quarré. Qu'il n'y a aucun triangle rectangle de
nombres dont l'aire soit un nombre quarré. La preuve se fait par
apagogeen   en cette manière. S'il y auoit aucun triangle rectangle
en nombres entiers, qui eust son aire esgale à un quarré, il y
auroit un autre triangle moindre que celuy la qui auroit la mesme
proprieté. S'il y en auoit un second moindre que le premier qui eust
la mesme proprieté il y en auroit par un pareil raisonnement un
troisieme moindre que ce second qui auroit la mesme proprieté et
enfin un quatrieme, un cinquieme etc. a l'infini en descendant. Or
est il qu'estant donné un nombre il n'y en a point infinis en
descendant moindres que celuy la, j'entens parler tousjours des
nombres entiers. D'ou on conclud qu'il est donc impossible qu'il y
ait aucun triangle rectange dont l'aire soit quarré. Vide foliu post
sequens....

Je fus longtemps sans pouvour appliquer ma methode aux questions
affirmatives, parce que le tour et le biais pour y venir est
beaucoup plus malaisé que celuy dont je me sers aux negatives. De
sorte que lors qu'il me falut demonstrer que tout nombre premier qui
surpasse de l'unité un multiple de 4, est composé de deux quarrez je
me treuvay en belle peine. Mais enfin une meditation diverses fois
reiterée me donna les lumieres qui me manquoient. Et les questions
affirmatives passerent par ma methods a l'ayde de quelques nouveaux
principes qu'il y fallust joindre par necessité. Ce progres de mon
raisonnement en ces questions affirmatives estoit tel. Si un nombre
premier pris a discretion qui surpasse de l'unité un multiple de 4
n'est point composé de deux quarrez il y aura un nombre premier de
mesme nature moindre que le donné; et ensuite un troisieme encore
moindre, etc. en descendant a l'infini jusques a ce que vous
arriviez au nombre 5, qui est le moindre de tous ceux de cette
nature, lequel il s'en suivroit n'estre pas composé de deux quarrez,
ce qu'il est pourtant d'ou on doit inferer par la deduction a
l'impossible que tous ceux de cette nature sont par consequent
composez de 2 quarrez.

Il y a infinies questions de cette espece. Mais il y en a quelques
autres que demandent de nouveaux principes pour y appliquer la
descente, et la recherche en est quelques fois si mal aisée, qu'on
n'y peut venir qu'avec une peine extreme. Telle est la question
suivante que Bachet sur Diophante avoüe n'avoir jamais peu
demonstrer, sur le suject de laquelle Mr. Descartes fait dans une de
ses lettres la mesme declaration, jusques la qu'il confesse qu'il la
juge si difficile, qu'il ne voit point de voye pour la resoudre.
Tout nombre est quarré, ou composé de deux, de trois, ou de quatre
quarrez. Je l'ay enfin rangée sous ma methode et je demonstre que si
un nombre donné n'estoit point de cette nature il y en auroit un
moindre que ne le seroit par non plus, puis un troisieme moindre que
le second etc. a l'infini, d'ou l'on infere que tous les nombres
sont de cette nature....

J'ay ensuit consideré questions que bien que negatives ne restent
pas de recevoir tres-grande difficulté, la methods pour y pratiquer
la descente estant tout a fait diverse des precedentes comme il sera
aisé d'espouver. Telles sont les suivantes. Il n'y a aucun cube
divisible en deux cubes. Il n'y a qu'un seul quarré en entiers que
augmenté du binaire fasse un cube, ledit quarré est 25. Il n'y a que
deux quarrez en entiers lesquels augmentés de 4 fassent cube,
lesdits quarrez sont 4 et 121....

Apres avoir couru toutes ces questions la plupart de diverses (sic)
nature et de differente façon de demonstrer, j'ay passé a
l'invention des regles generales pour resoudre les equations simples
et doubles de Diophante. On propose par exemple 2 quarr. + 7957
esgaux a un quarré (hoc est 2xx + 7967 \propto   quadr.) J'ay une
regle generale pour resoudre cette equation si elle est possible, on
decouvrir son impossibilité. Et ainsi en tous les cas et en tous
nombres tant des quarrez que des unitez. On propose cette equation
double 2x + 3 et 3x + 5 esgaux chaucon a un quarré. Bachet se
glorifie en ses commentaires sur Diophante d'avoir trouvé une regle
en deux cas particuliers. Je me donne generale en toute sorte de
cas. Et determine par regle si elle est possible ou non....

Voila sommairement le conte de mes recherches sur le sujet des
nombres. Je ne l'ay escrit que parce que j'apprehende que le loisir
d'estendre et de mettre au long toutes ces demonstrations et ces
methodes me manquera. En tout cas cette indication seruira aux
sçauants pour trouver d'eux mesmes ce que je n'estens point,
principlement si Mr. de Carcaui et Frenicle leur font part de
quelques demonstrations par la descente que je leur ay envoyees sur
le suject de quelques propositions negatives. Et peut estre la
posterité me scaure gré de luy avoir fait connoistre que les anciens
n'ont pas tout sceu, et cette relation pourra passer dans l'esprit
comme parle le grand Chancelier d'Angleterre, suivant le sentiment
et la devise duquel j'adjousteray, multi pertransibunt et augebitur
scientia.

(ii) I next proceed to mention Fermat's use in geometry of analysis
and of infinitesimals. It would seem from his correspondence that he
had thought out the principles of analytical geometry for himself
equation, or, as he calls it, the `specific property,'' of a curve
all its properties could be deduced. His extant papers on geometry
deal, however, mainly with the application of infinitesimals to the
determination of the tangents to curves, to the quadrature of curves,
and to questions of maxima and minima; probably these papers are a
revision of his original manuscripts (which he destroyed), and were
written about 1663, but there is no doubt that he was in possession of
the general idea of his method for finding maxima and minima as early
as 1628 or 1629.

He obtained the subtangent to the ellipse, cycloid, cissoid, conchoid,
and quadratrix by making the ordinates of the curve and a straight
line the same for two points whose abscissae were x and x - e; but
there is nothing to indicate that he was aware that the process was
general, it is probable that he never separated it, so to speak, from
the symbols of the particular problem he was considering. The first
definite statement of the method was due to Barrow, and was published
in 1669.

Fermat also obtained the areas of parabolas and hyperbolas of any
order, and determined the centres of mass of a few simple laminae and
of a paraboloid of revolution. As an example of his method of solving
these questions I will quote his solution of the problem to find the
area between the parabola y³ = p x², the axis of x, and the line x =
a. He says that, if the several ordinates of the points for which x
is equal to a, a(1 - e), a(1 - e)²,... be drawn, then the area will be
split into a number of little rectangles whose areas are respectively

ae(pa^2)^(1/3), ae(1-e) ( pa^2(1-e)^2 )^(1/3),... .

The sum of these is p^(1/3) a^(5/3) e / ( 1 - (1 - e)^(5/3) ) ; and by
a subsidiary proposition (for he was not acquainted with the binomial
theorem) he finds the limit of this, when e vanishes, to be (3/5)
p^(1/3) a^(5/3) . The theorems last mentioned were published only
after his death; and probably they were not written till after he had
read the works of Cavalieri and Wallis.

to and on either side of a maximum (or minimum) value must be equal.
Fermat applied this principle to a few examples. Thus, to find the
maximum value of x(a - x), his method is essentially equivalent to
taking a consecutive value of x, namely x - e where e is very small,
and putting x(a - x) = (x - e)(a - x + e). Simplifying, and ultimately
putting e = 0, we get x = ½. This value of x makes the given
expression a maximum.

(iii) Fermat must share with Pascal the honour of having founded the
theory of probabilities. I have already mentioned the problem proposed
to Pascal, and which he communicated to Fermat, and have there given
Pascal's solution. Fermat's solution depends on the theory of
combinations, and will be sufficiently illustrated by the following
example, the substance of which is taken from a letter dated August
24, 1654, which occurs in the correspondence with Pascal. Fermat
discusses the case of two players, A and B, where A wants two points
to win and B three points. Then the game will be certainly decided in
the course of four trials. Take the letters a and b, and write down
all the combinations that can be formed of four letters. These
combinations are 16 in number, namely, aaaa, aaab, aaba, aabb; abaa,
abab, abba, abbb; baaa, baab, baba, babb; bbaa, bbab, bbba, bbbb. Now
every combination in which a occurs twice or oftener represents a case
favourable to A, and every combination in which b occurs three times
or oftener represents a case favourable to B. Thus, on counting them,
it will be found that there are 11 cases favourable to A, and 5 cases
favourable to B; and since these cases are all equally likely, A's
chance of winning the game is to B's chance as 11 is to 5.

The only other problem on this subject which, as far as I know,
attracted the attention of Fermat was also proposed to him by Pascal,
and was as follows. A person undertakes to throw a six with a die in
eight throws; supposing him to have made three throws without success,
what portion of the stake should he be allowed to take on condition of
giving up his fourth throw? Fermat's reasoning is as follows. The
chance of success is 1/6, so that he should be allowed to take 1/6 of
the stake on condition of giving up his throw. But if we wish to
estimate the value of the fourth throw before any throw is made, then
the first throw is worth 1/6 of the stake; the second is worth 1/6 of
what remains, that is 5/36 of the stake; the third throw is worth 1/6
of what now remains, that is, 25/216 of the stake; the fourth throw is
worth 1/6 of what now remains, that is, 125/1296 of the stake.

Fermat does not seem to have carried the matter much further, but his
correspondence with Pascal shows that his views on the fundamental
principles of the subject were accurate: those of Pascal were not
altogether correct.

Fermat's reputation is quite unique in the history of science. The
problems on numbers which he had proposed long defied all efforts to
solve them, and many of them yielded only to the skill of Euler. One
still remains unsolved. This extraordinary achievement has
overshadowed his other work, but in fact it is all of the highest
order of excellence, and we can only regret that he thought fit to
write so little.
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