Spacetimes dressed with stealth electromagnetic fields

Stealth field configurations by definition have a vanishing energy-momentum tensor, thus do not contribute to the gravitational field equations. While only trivial fields can be stealth in Maxwell's electrodynamics, nontrivial stealth fields appear in some nonlinear models of electromagnetism. We find the necessary and sufficient conditions for the electromagnetic fields to be stealth and analyse which models admit such configurations. Furthermore, we present some concrete exact solutions, featuring a class of black holes dressed with the stealth electromagnetic hair, closely related to force-free solutions. Stealth hair does not alter the generalized Smarr formula, but may contribute to the Komar charges.Comment: 6 pages; reformatted and slightly revised published version, with an additional comment at the end of the Section II.

Constraints on the symmetry noninheriting scalar black hole hair

Any recipe to grow black hole hair has to circumvent no-hair theorems by violating some of their assumptions. Recently discovered hairy black hole solutions exist due to the fact that their scalar fields don't inherit the symmetries of the spacetime metric. We present here a general analysis of the constraints which limit the possible forms of such a hair, for both the real and the complex scalar fields. These results can be taken as a novel piece of the black hole uniqueness theorems or simply as a symmetry noninheriting Ans\"atze guide. In addition we introduce new classification of the gravitational field equations which might prove useful for various generalizations of the theorems about spacetimes with symmetries.Comment: 10 page

Multivariate Cointegration Technique Estimation of Health Demand Function: The Case of Croatia

In this paper multivariate Johansen cointegration technique is used in order to estimate health demand function in Croatia. Empirical estimate is based on the theoretical foundation of Grossman's model. According to the estimate, the number of physician visits in Croatia is a function of percentage of urban population, GDP, number of beds per 100 000 people, number of physicians per 100 000 people and total fertility rate. All estimated systems demonstrated strong feedbacks indicating number of endogenous variables greater than one.health demand function, health capital, Grossman's model, Croatian health care system, Johansen cointegration technique