Doctoral defence: Andre Ostrak “Diameter two properties in spaces of Lipschitz functions”

On 26 August at 14:00 Andre Ostrak will defend his doctoral thesis “Diameter two properties in spaces of Lipschitz functions” for obtaining the degree of Doctor of Philosophy (in Mathematics).

Supervisors:
Professor Rainis Haller, University of Tartu
Associate Professor Märt Põldvere, University of Tartu

Opponents:
Professor Antonín Procházka, University of Franche-Comté (France)
Professor Ramón J. Aliaga, Polytechnic University of Valencia (Spain) 

Summary
Geometric properties of Banach spaces, which imply that all specific subsets of the unit ball have diameter 2, are usually referred to as diameter 2 properties. For example, every slice of the unit ball of Banach spaces l_∞, C[0, 1] and L_1[0, 1] has diameter equal to 2. On the other hand, the unit ball of a reflexive Banach space or a separable dual space like l_1 has slices of arbitrarily small diameter. In the thesis, we study diameter 2 properties for the Banach spaces Lip_0(M) of Lipschitz functions f : M → R, where the norm of f is its Lipschitz constant and M is a metric space. The space Lip_0(M) is a dual space with a specific predual, the Lipschitz-free space F(M). The emphasis of the work is on the diameter 2 properties SSD2P and w*-SSD2P. In the thesis, we characterise the w*-SSD2P for Lip_0(M) in terms of the metric space M and via a property of the Lipschitz-free space F(M). We also study the latter property in the general setting of Banach spaces. The given characterisations correspond well with previously obtained characterisations of other diameter 2 properties for the space Lip_0(M). By using the de Leeuw’s transform, we obtain simple sufficient conditions for the space Lip_0(M) to have the following diameter 2 properties: the SSD2P, the SD2P, and the D2P. Our results show that these diameter 2 properties are common but still different in the class of Lipschitz functions spaces. This significantly improves upon and generalises the previously obtained results for special cases by different mathematicians. The de Leeuw’s transform is applicable in a more general context. For instance, we use it to answer a recent question about Daugavet-points in the space Lip_0(M).