Interacting strings on AdS_3

We consider string theory on AdS_3 in terms of the Wakimoto free field representation. The scattering amplitudes for N unitary tachyons are analysed in the factorization limit and the poles corresponding to the mass-shell conditions for physical states are extracted. The vertex operators for excited levels are obtained from the residues and their properties are examined. Negative norm states are found at the second mass level.Comment: 24 pages, 2 figures, LaTeX. Substantial changes: ghosts found, conclusions modified accordingly, references added. Final version to appear in JHE

Coulomb integrals and conformal blocks in the AdS3-WZNW model

We study spectral flow preserving four-point correlation functions in the AdS3-WZNW model using the Coulomb gas method on the sphere. We present a multiple integral realization of the conformal blocks and explicitly compute amplitudes involving operators with quantized values of the sum of their spins, i.e., requiring an integer number of screening charges of the first kind. The result is given as a sum over the independent configurations of screening contours yielding a monodromy invariant expansion in powers of the worldsheet moduli. We then examine the factorization limit and show that the leading terms in the sum can be identified, in the semiclassical limit, with products of spectral flow conserving three-point functions. These terms can be rewritten as the m-basis version of the integral expression obtained by J. Teschner from a postulate for the operator product expansion of normalizable states in the H3+-WZNW model. Finally, we determine the equivalence between the factorizations of a particular set of four-point functions into products of two three-point functions either preserving or violating spectral flow number conservation. Based on this analysis we argue that the expression for the amplitude as an integral over the spin of the intermediate operators holds beyond the semiclassical regime, thus corroborating that spectral flow conserving correlators in the AdS3-WZNW model are related by analytic continuation to correlation functions in the H3+-WZNW model.Comment: 28 pages; references modified, published versio

Looking for Simple Common Schemes to Design Recognizer P Systems with Active Membranes That Solve Numerical Decision Problems

Earlier solutions to decision problems by means of P systems used many counter objects to control the synchronization of different stages in a computation (usually as many counters as the stage must last in the worst case). In this paper we propose a way to replace those counters with some spacial objects for each stage. Furthermore, following the ideas presented in [1], in order to have a common scheme to attack numerical problems, all instances of a problem with the same size are solved by the same P system (which depends on the size) given an input which describes the corresponding instance of the problem. We illustrate these ideas with a cellular solution to the Subset-Sum problem

Teenage Childbearing in Latin American Countries

In spite of the rapid fertility transition experienced by most Latin American and Caribbean countries, teenage fertility has not changed at the same pace or in the same direction. Given that early childbearing is deleterious for both mother and child, we describe differentials in the levels and trends in teenage childbearing and analyze its proximate and socioeconomic determinants. We used Demographic and Health Surveys data from six LAC countries for which data are available for the second half of the 1990s: Bolivia, Brazil, Colombia, Guatemala, Dominican Republic and Peru. Teenage fertility trends indicate different patterns of change across countries by area of residence. However, in most countries teenage fertility has increased in rural areas but declined or remained constant in urban areas. Different contributions of marriage, proper use of family planning methods, and premarital births to teenage fertility behavior are reflected in differentials in unmarried parenthood across countries. Socioeconomic determinants are analyzed through simple logit model, multilevel analysis, and continuous-time hazard rate models. These analyses improve on prior research on LAC countries by including contextual/regional factors, isolating the effects into differentials in sexual activity and rates of childbearing, and by comparing the socioeconomic determinants of the timing of first birth and premarital birth. This research demonstrates that the effect of socioeconomic variables on the rate of childbearing can act through the timing of initial sexual intercourse (such as education, socioeconomic conditions of the households and area or residence) or through the timing of first birth (such as socialization in a female-headed family, availability - cceptability - use of family planning, and regional - country conditions, such as cultural and inherent characteristics).

TEENAGE CHILDBEARING IN LATIN AMERICAN COUNTRIES

In spite of the rapid fertility transition experienced by most LAC countries, teenage fertility has not changed at the same pace or direction. Given early childbearing is deleterious for both mother and child, we describe the differentials in the levels and trends in teenage childbearing and analyze its proximate and socioeconomic determinants. We used DHS data from six LAC countries, Bolivia, Brazil, Colombia, Guatemala, Dominican Republic and Peru, for which there is available data for the second half of the 90´s. Teenage fertility trends indicate different patterns of change across countries by area of residence. However, in most countries teenage fertility has increased in rural areas but has declined or remained constant in urban areas. Different contributions of marriage, proper use of family planning methods, and premarital births to teenage fertility behavior are reflected in differentials in unmarried parenthood across countries. Socioeconomic determinants are analyzed through: simple logit model, multilevel analysis, and continuous-time hazard rate models. These analyzes improve on prior research on LAC countries by: including contextual/regional factors, by isolating the effects into differentials in sexual activity and rates of childbearing, and by comparing the socioeconomic determinants of the timing of first birth and premarital birth. This research demonstrates that the effect of socioeconomic variables on the rate of childbearing can act through the timing of initial sexual intercourse (such as education, socioeconomic conditions of the households and area or residence) or through the timing of first birth (such as socialization in a female-headed family, availability/acceptability/use of family planning, and regional/country conditions - cultural and inherent characteristics).Childbearing

On the solvability of the Yakubovich linear-quadratic infinite horizon minimization problem

The Yakubovich Frequency Theorem, in its periodic version and in its general nonautonomous extension, establishes conditions which are equivalent to the global solvability of a minimization problem of infinite horizon type, given by the integral in the positive half-line of a quadratic functional subject to a control system. It also provides the unique minimizing pair \lq\lq solution, control\rq\rq~and the value of the minimum. In this paper we establish less restrictive conditions under which the problem is partially solvable, characterize the set of initial data for which the minimum exists, and obtain its value as well a minimizing pair. The occurrence of exponential dichotomy and the null character of the rotation number for a nonautonomous linear Hamiltonian system defined from the minimization problem are fundamental in the analysis.Ministerio de Economía y Competitividad / FEDER, MTM2015-66330-PMinisterio de Ciencia, Innovación y Universidades, RTI2018-096523-B-I00European Commission, H2020-MSCA-ITN-2014INDAM -- GNAMPA Project 201

Peer Victimization in Overweight Adolescents and Its Effect on Their Self-Esteem and Peer Difficulties

This study has three objectives: to examine whether adolescents who perceive themselves as overweight differ from others in terms of offline victimization at school, cybervictimization, self-esteem, and difficulties relating to peers; to examine the possible effects of offline and cybervictimization on self-esteem and difficulties relating to peers; and to examine the possible moderating role of perceiving oneself as overweight on those effects. Previously validated questionnaires were applied to a sample of 3145 adolescents in Asturias (Spain). Descriptive, inferential, correlational, and structural equation analyses were performed. Adolescents who perceived themselves as overweight reported being victims of both offline victimization and most forms of cybervictimization to a greater extent than those who did not perceive themselves as overweight. They also reported lower self-esteem and more peer difficulties (shyness or social anxiety). In both groups of adolescents, victimization and cybervictimization were correlated with each other, both types of victimization had direct, negative effects on self-esteem, and self-esteem in turn had a direct, negative effect on peer difficulties. Furthermore, offline victimization had a direct, positive effect on peer difficulties. Perceiving oneself as overweight moderated the effect of self-esteem on peer difficulties. In adolescents perceiving themselves as overweight, low self-esteem was a stronger risk factor of peer difficulties than in the rest of the adolescents. With high overall self-esteem there were no significant differences in peer difficulties between the adolescents perceiving themselves as overweight and the rest of the adolescents