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            Abstract Given a smooth projective variety over a number field and an elementof its Brauer group, we consider the specialization of the Brauerclass at a place of good reduction for the variety and the class. Weare interested in the case of K3 surfaces.We show that a Brauer class on a very general polarized K3 surfaceover a number field becomes trivial after specialization at a set ofplaces of positive natural density. We deduce that there exist cubic fourfolds over number fields that are conjecturally irrational, with rational reduction at a positive proportion of places. We also deduce that there are twisted derivedequivalent K3 surfaces which become derived equivalent after reductionat a positive proportion of places.more » « less
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            Balakrishnan, J.S.; Elkies, N.; Hassett, B.; Poonen, B.; Sutherland, A.V.; Voight, J. (Ed.)In this article, we discuss whether a single congruent number t can have two (or more) distinct corresponding triangles with the same hypotenuse. We describe and carry out computational experimentation providing evidence that this does not occur.more » « less
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            null (Ed.)We study rationality problems for smooth complete intersections of two quadrics. We focus on the three-dimensional case, with a view toward understanding the invariants governing the rationality of a geometrically rational threefold over a non-closed field. (with an appendix by Jean-Louis Colliot-Thélène)more » « less
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