1 Introduction
The problem of studying positive integers n which occur as areas of rational
right triangle was of interest to the Greeks. The congruent number problem
was first discussed systematically by Arab scholars of the tenth century.
By the way recall that a positive integer n is a congruent number if it equals
to the area of right triangle with rational sides.
Since tenth century, some well-known mathematicians have devoted considerable
energy of the congruent number problem. For example Euler showed
that n = 7 is a congruent number with sides of lenght 24
5 ,
35
12 and 337
60 . It is
750 Umm¨ ¨ ug¨uls¨um O˘¨g¨ut and Refik Keskin
known that Leonardo Pisano (Fibonacci) was challenged around 1220 by Johannes
of Palermo to find a rational right triangle of area 5. He found the
right triangle with sides of lenght 3
2 ,
20
3 and 41
6 . Notice that the definition of a
congruent number does not require the sides of the triangle to be integer, only
rational. While n = 6 is the smallest possible area of a right triangle with integer
sides of lenght 3,4,5 , n = 5 is the area of right triangle with rational sides
of lenght 3
2 ,
20
3 and 41
6 . So n = 5 is the smallest congruent number. In 1225,
Fibonacci wrote a general treatment about the congruent number problem, in
which he stated out without proof that if n is a perfect square then n cannot
be a congruent number. The proof of such a claim had to wait until Pierre de
Fermat. He showed that n = 1 and so every square number is not a congruent
number by using his method of infinite descent[6]. One can look at [4] and
[7] for Fermat’s descent method. In the present study we will show that if n
is a congruent number then n can not be a perfect square by using the same
method. Moreover, we proved Fermat’s last theorem for n = 4, which states
that the equation x4 + y4 = z4 has no solutions in positive integers.