In this dissertation, we study the following two topics, i.e., the union closed set conjecture and the maximum edges cut in connected digraphs. The union-closed-set-conjecture-topic goes as follows. A finite family of finite sets is union closed if it contains the union of any two sets in it. Let XF = ∪F∈FF . A union closed family of sets is separating if for any two distinct elements in F , th...

We study the renormalization of non-commutative gauge theories with matter. As in the scalar field theory cases, there are logarithmic infrared divergences resulting from integrating out high momentum modes. In order to reproduce the correct infrared behaviour, the Wilsonian effective action has to include certain ’‘closed string“ modes with prescribed couplings. In the case of quiver gauge the...

We consider the region of closed timelike curves (CTC’s) in three-dimensional flat Lorentz spacetimes. The interest in this global geometrical feature goes beyond the purely mathematical. Such spacetimes may be considered lower-dimensional toy models of sourceless Einstein gravity or cosmology. In three dimensions all such spacetimes are known: they are quotients of Minkowski space by a suitabl...