Linear Codes are Optimal for Index-Coding Instances with Five or Fewer Receivers

We study zero-error unicast index-coding instances, where each receiver must perfectly decode its requested message set, and the message sets requested by any two receivers do not overlap. We show that for all these instances with up to five receivers, linear index codes are optimal. Although this class contains 9847 non-isomorphic instances, by using our recent results and by properly categorizing the instances based on their graphical representations, we need to consider only 13 non-trivial instances to solve the entire class. This work complements the result by Arbabjolfaei et al. (ISIT 2013), who derived the capacity region of all unicast index-coding problems with up to five receivers in the diminishing-error setup. They employed random-coding arguments, which require infinitely-long messages. We consider the zero-error setup; our approach uses graph theory and combinatorics, and does not require long messages.Comment: submitted to the 2014 IEEE International Symposium on Information Theory (ISIT

Stringy Stability of Dilaton Black Holes in 5-Dimensional Anti-de Sitter Space

Flat electrical charged black holes in 5-dimensional anti-de Sitter space have been applied to the study of the phase diagram of quark matter via AdS/CFT correspondence. In such application it is argued that since the temperature of the quark gluon plasma is bounded away from zero, the dual black hole cannot be arbitrarily cold, but becomes unstable due to stringy instability once it reaches sufficiently low temperature. We study the stringy stability of flat dilaton black holes with dilaton coupling $\alpha=1$ in asymptotically anti-de Sitter space and show that unlike the purely electrically charged black hole, these dilaton black holes do not suffer from stringy instability.Comment: Published in Proceedings of the Conference in Honor of Murray Gell-Mann's 80th Birthday, p.583-590, World Scientific, 2010, Singapor

Hawking Evaporation Time Scale of Topological Black Holes in Anti-de Sitter Spacetime

It was recently pointed out that if an absorbing boundary condition is imposed at infinity, an asymptotically anti-de Sitter Schwarzschild black hole with a spherical horizon takes only a finite amount of time to evaporate away even if its initial mass is arbitrarily large. We show that this is a rather generic property in AdS spacetimes: regardless of their horizon topologies, neutral AdS black holes in general relativity take about the same amount of time to evaporate down to the same size of order L, the AdS length scale. Our discussion focuses on the case in which the black hole has toral event horizon. A brief comment is made on the hyperbolic case, i.e. for black holes with negatively curved horizons.Comment: Published versio