Història de la matemàtica. Grècia IIb (Els Elements d'Euclides, llibres VII, VIII, IX, X, XI, XII i XIII): Resultats, textos i contextos

Step into the profound logical architecture of ancient Greece, where the very foundations of number and space were meticulously laid bare. This journey delves into the latter half of Euclid's monumental "Elements," unraveling the intricate tapestry of thought that shaped centuries of mathematical understanding. We begin not with lines and circles, but with the very essence of number itself, exploring the properties and relationships of integers that form the bedrock of arithmetic.

Our exploration commences with Book VII, where the veil is lifted on the theory of numbers. Here, the definitions of unity, even, odd, and prime numbers are established, paving the way for profound insights into their behavior. One encounters the elegant dance of divisors and multiples, and witnesses the ingenious method of antanaresis - now famously known as the Euclidean algorithm - unveiled for determining the greatest common divisor of two or more numbers. It is a world where numerical ratios and proportions are rigorously examined, building a coherent framework for understanding the discrete quantities of the cosmos.

The journey into number theory continues through Book VIII, where the focus shifts to numbers in continued proportion, the very concept we now recognize as geometric sequences. Here, the properties of square numbers and cube numbers are meticulously explored, demonstrating how these special magnitudes fit within the grand scheme of proportional relationships. The careful arrangement of these propositions reveals a deep understanding of numerical progression and the inherent order within sets of numbers.

Book IX then brings us to some of the most celebrated theorems of antiquity. It is within these pages that the infinitude of prime numbers is irrefutably demonstrated, a revelation that continues to inspire awe. The discourse extends to the fascinating characteristics of odd and even numbers, culminating in the elegant identification of perfect numbers - those rare integers equal to the sum of their proper divisors. The pursuit of these numerical truths unveils the hidden symmetries and profound depths of arithmetic.

A significant shift occurs with Book X, a dense and challenging exploration into the realm of incommensurable magnitudes, what we now call irrational numbers. This book grapples with quantities that cannot be expressed as a ratio of two integers, such as the diagonal of a square in relation to its side. It classifies these "irrational" segments into various types, a testament to the Greeks' sophisticated understanding of numbers beyond the rational. This intricate classification, often drawing upon the work of Theaetetus, represents a monumental effort to tame the seemingly unruly aspects of continuous quantities.

Transitioning from the abstract world of numbers, Books XI, XII, and XIII plunge us into the majesty of three-dimensional space, laying the groundwork for solid geometry. Book XI introduces the fundamental propositions of solids, defining what it means for a figure to possess length, breadth, and depth. It meticulously examines the relationships between planes and lines in space, the intersections they form, and the properties of basic three-dimensional figures like parallelepipeds, pyramids, prisms, and spheres. One begins to perceive the world not just as flat surfaces, but as volumes and forms, carefully constructed and defined.

With Book XII, we witness the power of Eudoxus's method of exhaustion, a brilliant precursor to integral calculus. This revolutionary technique allows for the calculation of areas and volumes by approximating them with an ever-increasing number of simpler figures. It is here that one finds rigorous proofs demonstrating that the areas of circles are to one another as the squares of their diameters, and that the volumes of spheres are to one another as the cubes of their diameters. The method of exhaustion, applied with unparalleled precision, reveals the quantitative relationships governing cones, pyramids, cylinders, and spheres, treating them as limits of polygons and polyhedra.

Finally, Book XIII culminates the entire "Elements" with a breathtaking exposition on the five regular Platonic solids: the tetrahedron (pyramid), cube (hexahedron), octahedron, dodecahedron, and icosahedron. This book provides the constructions of these perfect polyhedra, demonstrating how each can be inscribed within a given sphere. It meticulously compares the ratios of their edges to the radius of the circumscribing sphere and concludes with the profound proof that only these five regular solids can exist. This grand finale synthesizes the geometric and arithmetic principles explored throughout the "Elements," offering a timeless vision of harmony and order in the mathematical universe.