1.1. Elliptic curves 3
For instance, the number 6 is congruent, because the right triangle
with sides of length (a, b, c) = (3,4,5) has area equal to
= 6.
Similarly, the number 30 is the area of the right triangle with sides
(5,12,13); thus, 30 is a congruent number.
Figure 1. A right triangle of area 5 and rational sides.
The number 5 is congruent but there is no right triangle with
integer sides and area equal to 5. However, our definition allowed
rational sides, and the triangle with sides
has area exactly
5. We do not allow, however, triangles with irrational sides even if
the area is an integer. For example, the right triangle (1,2,

5) has
area 1, but that does not imply that 1 is a congruent number (in fact,
1 is not a congruent number, as we shall see below).
The congruent number problem is one of the oldest open problems
in number theory. For more than a millennium, mathematicians have
attempted to provide a characterization of all congruent numbers.
The oldest written record of the problem dates back to the early
Middle Ages, when it appeared in an Arab manuscript written before
972 (a later 10th century manuscript written by Mohammed Ben
Alcohain would go as far as to claim that the principal object of the
theory of rational right triangles is to find congruent numbers). It is
known that Leonardo Pisano, a.k.a. Fibonacci, was challenged around
1220 by Johannes of Palermo to find a rational right triangle of area
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