O Teorema Egregium

Abstract

During the development of differential geometry around the 17th century, an old problem occupied the minds of mathematicians at the time, which was determining whether the so-called 5th postulate of Euclid was in fact a postulate or a theorem. This postulate, which had an equivalent version published in 1795 by John Playfair (1748–1819), says that: through a point outside a given straight line it is possible to draw a single straight line parallel to the given straight line". There were many attempts to "prove"the fifth postulate, all of which failed. The answer to this question was given years later by Gauss, Lobachevski and Bolyai. In their work Disquisitiones generales circa superficies curves, Gauss shows that the curvature K(p) of a surface at the point p, initially calculated through the determinant of the differential of dNp which depends on the socalled first and second fundamental forms, actually depends only on the coefficients of the first fundamental form and their derivatives, and can be calculated using a formula that bears his name, the so-called Gauss formula. As a consequence of this formula we have the so-called Egregium Theorem which states that the Gaussian curvature of a surface is an invariant intrinsic, that is, it does not depend on the environment the surface is in and consequently, it is invariant due to local isometries. This discovery is closely related to non-Euclidean geometries, since the geometry of a surface with non-zero curvature is non-Euclidean. A consequence of this fact is that the 5th postulate is in fact a postulate and not a theorem. In this work, we will study the concepts necessary to understand Gauss’s Egregium theorem and its demonstration, as well as some applications of this important result.