By Luciano Boi, Dominique Flament, Jean-Michel Salanskis

Those innocuous little articles aren't extraordinarily priceless, yet i used to be brought on to make a few comments on Gauss. Houzel writes on "The start of Non-Euclidean Geometry" and summarises the proof. primarily, in Gauss's correspondence and Nachlass you will find proof of either conceptual and technical insights on non-Euclidean geometry. maybe the clearest technical result's the formulation for the circumference of a circle, k(pi/2)(e^(r/k)-e^(-r/k)). this can be one example of the marked analogy with round geometry, the place circles scale because the sine of the radius, while the following in hyperbolic geometry they scale because the hyperbolic sine. nonetheless, one needs to confess that there's no proof of Gauss having attacked non-Euclidean geometry at the foundation of differential geometry and curvature, even supposing evidently "it is hard to imagine that Gauss had no longer noticeable the relation". by way of assessing Gauss's claims, after the guides of Bolyai and Lobachevsky, that this used to be identified to him already, one may still probably do not forget that he made comparable claims concerning elliptic functions---saying that Abel had just a 3rd of his effects and so on---and that during this example there's extra compelling proof that he used to be basically correct. Gauss exhibits up back in Volkert's article on "Mathematical development as Synthesis of instinct and Calculus". even if his thesis is trivially right, Volkert will get the Gauss stuff all incorrect. The dialogue issues Gauss's 1799 doctoral dissertation at the basic theorem of algebra. Supposedly, the matter with Gauss's facts, that is purported to exemplify "an development of instinct with regards to calculus" is that "the continuity of the airplane ... wasn't exactified". in fact, somebody with the slightest knowing of arithmetic will comprehend that "the continuity of the airplane" is not any extra a subject during this evidence of Gauss that during Euclid's proposition 1 or the other geometrical paintings whatever in the course of the thousand years among them. the genuine factor in Gauss's evidence is the character of algebraic curves, as in fact Gauss himself knew. One wonders if Volkert even afflicted to learn the paper due to the fact he claims that "the existance of the purpose of intersection is taken care of by way of Gauss as anything completely transparent; he says not anything approximately it", that is it appears that evidently fake. Gauss says much approximately it (properly understood) in a protracted footnote that indicates that he acknowledged the matter and, i might argue, acknowledged that his facts was once incomplete.

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**Extra resources for 1830-1930: A Century of Geometry: Epistemology, History and Mathematics (English and French Edition)**

**Example text**

If C denotes the midpoint of AB, we have OA · OB = 2 Thus, O B = 2 sin ψ sin ϕ [O AC ] 2 sin ψ [O AB] =4 = O A · OC . sin ψ sin ϕ sin ϕ OC . Similarly, O A = OA · OB = 2 sin(ϕ−ψ) sin ϕ OC , so 4 sin ψ sin(ϕ − ψ) 2 OC . sin2 ϕ The volume of O ABC is proportional to the product O A · O B · OC, and the identity above shows that it is proportional to OC · OC 2 . The problem now is to ﬁnd the lines CC through M such that C is on r, C is on r , and OC · OC 2 is a minimum. 3 that there exists exactly one line with this property and it is such that C M : C M = 2 : 1.

54 Chapter 1. Methods for Finding Geometric Extrema The level curve L c is easily determined using the fact that the sum of distances from an arbitrary point on the base of an isosceles triangle to the other two sides of the triangle is constant. Example 7 Now we consider two important curves in the plane: the ellipse and the hyperbola. Let A and B be given points in the plane. Consider the functions f (M) = M A + M B and g(M) = |M A − M B|. The level curves of f (M) are called ellipses, while these of g(M) are called hyperbolas.

Hence ≥ 12 and = 12 when t = 16. m. ♠ The following problem gives a mathematical explanation of the law of Snell– Fermat, well known in physics, concerning the motion of light in an inhomogeneous medium. 2 A line is given in the plane and two points A and B on different sides of the line. A particle moves with constant speed v 1 in the half-plane containing A and with constant speed v 2 in the half-plane containing B. Find the path from A to B that is traversed in minimal time by the particle. Solution.