A covariant theory of gravitation in flat space-time is stated and compared with general relativity. The results of the theory of gravitation in flat space-time and of general relativity agree for weak gravitational fields to low approximations. For strong fields the results of the two theories deviate from one another. Flat space-time theory of gravitation gives under some natural assumptions ...

Fix an algebraic space S, and let X and Y be separated Artin stacks of finite presentation over S with finite diagonals (over S). We define a stack HomS(X ,Y) classifying morphisms between X and Y. Assume that X is proper and flat over S, and fppf–locally on S there exists a finite finitely presented flat cover Z → X with Z an algebraic space. Then we show that HomS(X ,Y) is an Artin stack with...

In the theory of schemes, faithfully flat descent is a very powerful tool. One wants a descent theory not only for quasi-coherent sheaves and morphisms of schemes (which is rather elementary), but also for geometric objects and properties of morphisms between them. In rigid-analytic geometry, descent theory for coherent sheaves was worked out by Bosch and Görtz [BG, 3.1] under some quasi-compac...

In GRB 051221A, an X-ray afterglow flat segment lasting ∼ 10 seconds represents the first clear case of strong energy injection in the external shock of a short GRB afterglow. In this work, we show that a millisecond pulsar with dipole magnetic field ∼ 10 Gauss could well account for that energy injection. The good quality X-ray flat segment thus suggests that the central engine of this short b...

The effect of imperfections on surface critical properties is studied for Ising models with nearest– neighbour ferromagnetic couplings on simple cubic lattices. In particular, results of Monte Carlo simulations for flat, perfect surfaces are compared to those for flat surfaces with random, ’weak’ or ’strong’, interactions between neighbouring spins in the surface layer, and for surfaces with st...

Kleinewillinghöfer classified Laguerre planes with respect to central automorphisms. Polster and Steinke investigated flat Laguerre planes and their so-called Kleinewillinghöfer types, that is, the Kleinewillinghöfer types with respect to the full automorphism group. For some of these types the existence question remained open. We provide strong necessary existence conditions for flat Laguerre ...

We generalize the strong Hanani-Tutte theorem to clustered graphs with two disjoint clusters, and show that an extension of our result to flat clustered graphs with three disjoint clusters is not possible. We also give a new and short proof for a result by Di Battista and Frati about efficient c-planarity testing of an embedded flat clustered graph with small faces based on the matroid intersec...