Type:

Doctoral Thesis 
Title:

Gauss's theorem on sums of 3 squares, sheaves, and Gauss composition

Author:

Gunawan, Albert

Publisher:

Mathematisch Instituut , Faculty of Science, Leiden University 
Issue Date:

20160308 
Keywords:

Cohomological intepretation of Gauss's theorem Gauss composition law 
Abstract:

Gauss's theorem on sums of 3 squares relates the number of primitive integer points on the sphere of radius the square root of n with the class number of some quadratic imaginary order. In 2011, Edixhoven sketched a different proof of Gauss's theorem by using an approach from arithmetic geometry. He used the action of the special orthogonal group on the sphere and gave a bijection between the set of SO_3(Z)orbits of such points, if nonempty, with the set of isomorphism classes of torsors under the stabilizer group. This last set is a group, isomorphic to the group of isomorphism classes of projective rank one modules over the ring Z[1/2,sqrt{n}]. This gives an affine space structure on the set of SO_3(Z)orbits on the sphere.
In Chapter 3 we give a complete proof of Gauss's theorem following Edixhoven's work and a new proof of Legendre's theorem on the existence of a primitive integer solution of the equation x^2+y^2+z^2=n by sheaf theory. In Chapter 4 we make the action given by the sheaf method of the Picard group on the set of SO_3(Z)orbits on the sphere explicit, in terms of SO_3(Q). 
Description:

Promotor: Bas Edixhoven, Promotor: Qing Liu With summaries in Dutch and French

Faculty:

Faculteit der Wiskunde en Natuurwetenschappen

Citation:

Gunawan, A., 2016, Doctoral Thesis, Leiden University 
Sponsor:

ALGANT Erasmus Mundus doctorate program 
Handle:

http://hdl.handle.net/1887/38431 

