Bird-like fossil footprints from the Late Triassic Melchor, Ricardo Nestor; de Valais, Silvina; Genise, Jorge Fernando The study of fossilized footprints and tracks of dinosaurs and other vertebrates has provided insight into the origin, evolution and extinction of several major groups and their behaviour; it has also been an important complement to their body fossil record1-4. The known history of birds starts in the Late Jurassic epoch (around 150 Myr ago) with the record of Archaeopteryx5, whereas the coelurosaurian ancestors of the birds date back to the Early Jurassic6. The hind limbs of Late Triassic epoch theropods lack osteological evidence for an avian reversed hallux and also display other functional differences from birds7. Previous references to suggested Late Triassic to Early Jurassic bird-like footprints have been reinterpreted as produced by non-avian dinosaurs having a high angle between digits II and IV8, 9 and in all cases their avian affinities have been challenged10. Here we describe well-preserved and abundant footprints with clearly avian characters from a Late Triassic redbed sequence of Argentina11, 12, at least 55 Myr before the first known skeletal record of birds. These footprints document the activities, in an environment interpreted as small ponds associated with ephemeral rivers, of an unknown group of Late Triassic theropods having some avian characters.

Estimation of Weibull parameters for the flexural strength of PMMA-based bone cements Riccardi, Carmen Cristina; Vallo, Claudia Ines The wide scatter of data observed in the strength of bone cements based on poly(methyl methacrylate) (PMMA) can be described properly by the two-parameter Weibull function. However, the statistical character of the distribution leads to an uncertainty in the parameters evaluated from a limited number of experiments. This study is concerned with the analysis of the methods of estimation as well as sample size on the estimates of the Weibull parameters. The maximum likelihood method, moments method, and linear regression method were studied. Monte Carlo simulations were carried out in order to assess the influence of the number of specimens tested on the Weibull parameters calculated by the different methods. The number of specimens tested displayed a large influence upon the calculated Weibull modulus. By applying weighing factors to the linear regression method, the standard deviation of Weibull parameters decreased significantly. As a compromise between minimizing both the dispersion of the evaluation method and the experimental effort, the use is suggested of the linear regression method with a minimum number of 20 specimens in a nonweighted analysis and 10 in a weighted analysis.

Symptoms of eutrophication in intertidal macroalgal assemblages of Nuevo Gulf (Patagonia, Argentina) Díaz, P.; López Gappa, Juan José; Piriz, Maria Luz The city of Puerto Madryn, located within a section of Nuevo Gulf (Patagonia, Argentina) known as Nueva Bay, can be regarded as an important centre of anthropogenic impact. Intertidal assemblages of macroalgae were studied in order to analyse whether eutrophication symptoms could be detected in the benthos. Three rocky shores showing similar characteristics were selected. One of these was located near the sewage outfall of the city of Puerto Madryn. Two control shores were chosen approximately 30 km northwards and southwards of the latter. Four seasonal surveys were performed between June 1998 and April 1999. Biomass data of 35 macroalgal species were analysed by diversity and evenness indices, non-Metric Multidimensional Scaling, and the ANOSIM permutation test. Species responsible for the differences between shores were identified by the SIMPER routine. The macroalgal assemblage of the impacted site was significantly different from those of the two control shores throughout the year. Ulva rigida, Corallina officinalis and Ralfsia sp. were the species mostly responsible for these differences. Results indicate that the impacted shore showed signs consistent with an intermediate degree of eutrophication, particularly the increased abundance of the opportunist green alga Ulva rigida.

Lithium and KB-R7943 effects on mechanics and energetics of rat heart muscle Bonazzola, Patricia; Egido, P.; Marengo, Fernando Diego; Savio Galimberti, E.; Ponce Hornos, Jorge Emilio The role of calcium influx on energy expenditure during cardiac contraction was studied. For this purpose; the described ability of lithium and KB-R 7943 (KBR) to diminish Ca entry through Na-Ca exchanger (Ponce-Hornos & Langer, J Mol Cell Cardiol 1980, 12, 1367, Satoh et al., Circulation 2000, 101, 1441) were used. In isolated contractions (contractions elicited after at least 5 min of rest) LiCl 45 mmol L -1 decreased pressure developed and pressure-time integral from 42.3 ± 2.7 and 14.5 ± 1.2 to 32.1 ± 3.4 mN mm -2 and 8.3 ± 0.9 mN mm -2 s, respectively. A similar effect was observed in regular contractions (at 0.16 Hz stimulation). The presence of KBR (5 μmol L -1) in the perfusate induced a slight but not significant decrease in pressure developed and pressure-time integral in steady-state contractions. As it was previously described, the heat involved in a heart muscle contraction can be decomposed into several components (H 1, H 2, H 3 and H 4), but only one (H 3) was associated with force generation. While H 3 decreased with lithium in both types of contractions, H 3/PtI ratio remained unaltered, indicating that the economy for pressure maintenance was unaffected. To further investigate the role of Ca entry on force development, a condition in which the contraction is mainly dependent on extracellular calcium was studied. An 'extra' stimulus applied 200 ms after the regular one in a muscle stimulated at 0.16 Hz induces a contraction with this characteristic (Marengo et al., Am J Physiol 1999, 276, H309). Lithium induced a strong decrease in pressure-time integral and H 3 associated with this contraction (43 and 45%, respectively) with no change in H 3/PtI ratio. Lithium also reduced (53%) an energy component (H 2) associated with Ca cycling. The use of KBR showed qualitatively similar results [i.e. a 33% reduction in pressure-time integral associated with the extrasystole (ES) with no changes in H 3/PtI ratio and a 30% reduction in) the H 2 component]. Li and KBR effects appear to be additive and in the presence of 45 mmol L -1 Li and 5 μmol L -1 KBR the extrasystole was abolished in 77%. Lithium and KBR effects particularly for the extrasystole can be explained through the inhibition of Ca entry via Na-Ca exchange giving support to the participation of the Na-Ca exchanger in the Ca influx from the extracellular space. In addition, the results also suggest the possibility of an effect of Li on an additional Ca sensitive locus (different than the Na-Ca exchanger). In this connection, in isolated contractions lithium decreased the energy release fraction related to mitochondrial processes (H 4) increasing the economy of the overall cardiac contraction.

The Magnetic Helicity Injected by Shearing Motions Démoulin, Pascal; Mandrini, Cristina Hemilse; van Driel Gesztelyi, Lidia; Lopez Fuentes, Marcelo Claudio; Aulanier, G. Photospheric shearing motions are one of the possible ways to inject magnetic helicity into the corona. We explore their efficiency as a function of their particular properties and those of the magnetic field configuration. Based on the work of M. A. Berger, we separate the helicity injection into two terms: twist and writhe. For shearing motions concentrated between the centers of two magnetic polarities the helicity injected by twist and writhe add up, while for spatially more extended shearing motions, such as differential rotation, twist and writhe helicity have opposite signs and partially cancel. This implies that the amount of injected helicity can change in sign with time even if the shear velocity is time independent. We confirm the amount of helicity injected by differential rotation in a bipole in the two particular cases studied by DeVore (2000), and further explore the parameter space on which this injection depends. For a given latitude, tilt and magnetic flux, the generation of helicity is slightly more efficient in young active regions than in decayed ones (up to a factor 2). The helicity injection is mostly affected by the tilt of the AR with respect to the solar equator. The total helicity injected by shearing motions, with both spatial and temporal coherence, is at most equivalent to that of a twisted flux tube having the same magnetic flux and a number of turns of 0.3. In the solar case, where the motions have not such global coherence, the injection of helicity is expected to be much smaller, while for differential rotation this maximum value reduces to 0.2 turns. We conclude that shearing motions are a relatively inefficient way to bring magnetic helicity into the corona (compared to the helicity carried by a significantly twisted flux tube).

Characterization of FeMCM-41 and FeZSM-5 catalysts to styrene production Bispo, J. R. C.; Oliveira, A. C.; Corrêa, M. L. S.; Fierro, J. L. G.; Marchetti, Sergio Gustavo; Rangel, Maria do Carmo FeMCM-41 and FeZSM-5 catalysts have been prepared and tested in the dehydrogenation of ethylbenzene to produce styrene. These new catalytic systems have high specific areas and can stabilize the trivalent state of iron. It was found that the FeZSM-5 catalysts are more active and selective than the FeMCM-41 ones and this behavior is explained in terms of the higher amount of the active oxidation state (Fe 3+) on the catalyst surface. These catalysts also result more active and selective than Fe203 (hematite) and are less toxic than the chromium-containing commercial catalysts used for this purpose.

Cold season synoptic scale waves over Subtropical South America Vera, Carolina Susana; Viglirarolo, Paula K.; Berbery, Ernesto Hugo The most active winter synoptic-scale wave patterns over South America are identified using an extended empirical orthogonal function (EEOF) technique and are physically diagnosed using composite methods. Results show that the leading modes of short timescale variability propagate along two main paths: over the subtropical jet latitudes (;308S) and over the subpolar jet latitudes (;608S). This research focuses on the subtropical mode and its evolution over South America. The observed structure of the systems associated with the subtropical mode resembles that of midlatitude baroclinic waves. Both cyclonic and anticyclonic perturbations display significant modifications in their threedimensional structure as they evolve over extratropical and subtropical South America. While the upper-level perturbations are mostly unaffected when moving eastward, the lower-level perturbations advance following the shape of the Andes Mountains and exhibit an abrupt equatorward migration at the lee side of the mountains. As a result of such detachment, smaller eddy heat fluxes are observed in the vicinity of the orography and consequently a weaker eddy baroclinic growth is observed. Once the upper-level system is on the lee side, the perturbations acquire a more typical baroclinic wave structure and low-level intensification of the system occurs. The latter is largest around 1000 km east of the orography, where enhanced moisture transports from tropical latitudes along the eastern portion of the low-level cyclone favor precipitation occurrence over southeastern South America. Those precipitation processes seem to provide a diabatic source of energy that further contributes to the strengthening of the low-level cyclone. In addition, an intensification of the cyclone once over the ocean was found in 60% of the situations considered, which is consistent with previous research suggesting an additional source of moisture and heat flux due to the warm waters of the Brazil Current.

LS 5039: A runaway microquasar ejected from the galactic plane Ribó, Marc; Paredes, Josep Maria; Romero, Gustavo Esteban; Benaglia, Paula; Martí, Josep; Fors, O.; García Sánchez, J. We have compiled optical and radio astrometric data of the microquasar LS 5039 and derived its proper motion. This, together with the distance and radial velocity of the system, allows us to state that this source is escaping from its own regional standard of rest, with a total systemic velocity of about 150 km/s and a component perpendicular to the galactic plane larger than 100 km/s. This is probably the result of an acceleration obtained during the supernova event that created the compact object in this binary system. We have computed the trajectory of LS 5039 in the past, and searched for OB associations and supernova remnants in its path. In particular, we have studied the possible association between LS 5039 and the supernova remnant G016.8-01.1, which, despite our efforts, remains dubious. We have also discovered and studied an HI cavity in the ISM, which could have been created by the stellar wind of LS 5039 or by the progenitor of the compact object in the system. Finally, in the symmetric supernova explosion scenario, we estimate that at least 17 solar masses were lost in order to produce the high eccentricity observed. Such a mass loss could also explain the observed runaway velocity of the microquasar.

Multilayer adsorption with multisite occupancy: An improved isotherm for surface characterization Riccardo, Jose Luis; Ramirez Pastor, Antonio Jose; Romá, Federico José In this work, an improved solution for multilayer adsorption of linear species is presented. The particular case of multilayer dimer adsorption is dealt with in detail, and a new adsorption isotherm is obtained for determination of surface area and adsorption energy from experiments. From the comparison between the new isotherm and the standard Brunauer-Emmett-Teller (BET) formalism, it arises that the monolayer volume, vm (or the surface area, A), can be up to 1.5 times larger than the one from the BET model. The proposed model is simple and easy to apply in practice and leads to new values of surface area and adsorption heat. Physically, these advantages are a consequence of properly considering the configurational entropy of the adsorbate.

Educar a la mujer en la Patagonia: las Hijas de María Auxiliadora y las imágenes femeninas (1880-1934) Nicoletti, Maria Andrea Nos proponemos analizar las imágenes femeninas a través del rol misionero y educativo de las Hijas de María Auxiliadora que fueron misioneras en la Patagonia desde 1880, en la formación de jóvenes en Junín de los Andes y en su relación con las indígenas en Tierra del Fuego. Como aporte a la historia del género, la imagen de la mujer en la Patagonia de fines del siglo XIX y XX es en general desconocida y poco investigada, fundamentalmente en relación a la construcción cultural del rol femenino en la historia de la educación que llevaron a cabo las religiosas como docentes y misioneras. A lo largo de este trabajo intentaremos analizar cómo las Hermanas no sólo generaron su propia imagen femenina, sino que la proyectaron en la formación y educación de niñas y mujeres, tanto migrantes como indígenas, que vivieron en una conflictiva sociedad de frontera como la patagónica.; L’articolo affronta lo studio dell’immagine della donna nelle fonti edite e inedite che si riferiscono al ruolo educativo-missionario delle Figlie di Maria Ausiliatrice che operarono in Patagonia dal 1880 sia nella formazione di giovani donne a Junín de los Andes, sia nelle loro relazioni con le donne indigene della Terra del Fuoco. L’immagine della donna nella Patagonia di fine secolo XIX e inizio del XX è in genere poco conosciuta e non adeguatamente studiata, soprattutto per quanto riguarda il contributo dato dalle religiose educatrici e missionarie alla promozione delle donne. L’articolo, che può offrire un significativo apporto alla storia di genere, evidenzia non solo l’autocoscienza femminile delle religiose, ma anche la proiezione di tale autocoscienza nell’educazione di bambine e ragazze tanto immigrate che indigene che vissero in una zona di frontiera non priva di conflitti quale quella della Patagonia.; The article deals with a study of the image of woman in published and unpublished works on the role of the FMA as educators and missionaries in Patagonia (where they have worked since 1880) in the formation of young women in Junín de los Andes as well as in their rapport with the native women of the “Tierra del Fuego”. Not much is known about the image of woman in Patagonia at the end of the 19th century and the beginning of the 20th. Neither is it a topic that has been studied in depth, especially the contribution of women religious educators and missionaries to the upliftment of women. The article, which can offer a significant contribution to gender history, highlights not only the self-awareness of religious sisters as women, but also how they projected it in the education of the immigrant and native girls of Patagonia, a frontier land not without its share of conflicts.; L’article affronte l’étude de l’image de la femme présente dans les sources éditées et inédites se référant au rôle éducatif et missionnaire des Filles de Marie Auxiliatrice qui ont travaillé à la Patagonie à partir de 1880 tant au niveau de la formation des jeunes filles à Junin de los Andes, que dans leurs relations avec les femmes indigènes de la Terre de Feu. L’image de la femme de la Patagonie de la fin du XIXème siècle et du début du XXème est en général peu connue et étudiée d’une façon inadéquate, surtout en ce qui concerne l’apport des religieuses éducatrices et missionnaires à la promotion des femmes L’article, pouvant être considéré une contribution significative à l’histoire du genre, met en relief non seulement l’auto conscience féminine des religieuses, mais aussi la projection de cette auto conscience dans l’éducation des enfants et des jeunes filles immigrées et indigènes de cette zone de frontière souvent en conflits.; Artykuł poswiecony jest badaniu zagadnienia obrazu kobiety jaki wyłania sie ze zródeł, zarówno tych wydanych juz drukiem jak i tych jeszcze nie opublikowanych, dotycza cych roli wychowawczo-ewangelizacyjnej Córek Maryi Wspomozycielki, działaja˛cych od 1880 roku w Patagonii, i zaangazowanych tak w dzieło formacji młodych kobiet w Junin de los Andes, jak i w nawiazywanie relacji z tubylczymi kobietami na Ziemi Ognistej. Ogólnie rzecz biorac, obraz patagonskiej kobiety z przełomu XIX i XX wieku, jest mało znany i nie zawsze pogłebiany od strony metodologicznej w odpowiedni sposób, szczególnie jezeli chodzi o wkład, jakie w promocje kobiety włozyły zakonnice-wychowawczynie i misjonarki. Artykuł, który moze byc znaczacym wkładem w poznanie historii kobiet jako kobiet, podkresla nie tylko samoswiadomosc kobieca konsekrowanych kobiet, ale przekazywanie tej samoswiadomosci poprzez sztuke wychowania równiez dziewczetom, pochodzacym zarówno z rodzin emigrantów jak i rodzin tubylczych, osiedlonych na tych przygranicznych i konfliktowych terytoriach jakimi była ówczesna Patagonia.

Surface roughness dependence of noncollinear phase matching Martínez Matos, Óscar; Torchia, Gustavo Adrian; Vaveliuk, Pablo; Bilmes, Gabriel Mario; Tocho, Jorge Omar Spontaneous noncollinear second harmonic generation in potassium dihydrogen phosphate (KDP) crystals was studied theoretically and experimentally as a function of the scattering centres on the surface of the crystal. A model that explains the main features of the second harmonic generated beam was developed in terms of the surface roughness defined by the size of the dispersing grains. It predicts, for the first time, the intensity distribution of the hollow cones obtained in noncollinear phase matching.

Unique effects of different fatty acid species on the physical properties of the Torpedo acetylcholine receptor membrane Antollini, Silvia Susana; Barrantes, Francisco Jose To study the effects produced by free fatty acids (FFA) on the biophysical properties of Torpedo marmorata nicotinic acetylcholine receptor-rich native membranes and to investigate the topology of their binding site(s), fluorescence measurements were carried out using the fluorescent probe Laurdan (6-dodecanoyl-2-(dimethyl-amino) naphthalene) and ADIFAB, an Acrylodan-derivatized intestinal fatty acid-binding protein. The generalized polarization (GP) of the former probe was used to learn about the physical state of the membrane upon FFA binding. Saturated FFA induced a slight increase in GP, whereas cis-unsaturated fatty acids decreased GP. Double bond isomerism could also be distinguished; oleic acid (18:1cis) induced a net disordering effect, whereas elaidic acid (18:1trans) produced no changes in GP. The changes in the efficiency of the Förster energy transfer from the protein to Laurdan brought about by addition of FFA, together with the distances involved in this process, indicate that all FFA studied share a common site at the lipid-protein interface. However, despite being located at the same site, each class of FFA differs in its effect on the physical properties of the membrane. These data lead us to suggest that it is the direct action of FFA at the lipid-protein interface, displacing essential lipids from their sites rather than changes in bulk properties such as membrane fluidity that accounts for the effect of FFA on the acetylcholine receptor membrane.

Las Escorias y Tierras Cocidas de la Pampa Zárate, Marcelo Arístides; Schultz, Peter Los enigmáticos fragmentos vítreos asociados a otros parecidos ladrillos que se encuentran en los sedimentos de la cercanos a Mar del Plata se deben al impacto de ocurrido hace 3,3 millones de años

Embryonal mass and hormone-associated effects of pregnancy inducing a differential growth of four murine tumors Bustuoabad, Oscar David; di Gianni, Pedro D.; Franco, Marcela; Kordon, Edith Claudia; Vanzulli, Silvia I.; Meiss, Roberto P.; Grion, Lorena C.; Diaz, Graciela Susana; Nosetto, Sergio H.; Hockl, Pablo Francisco; Lombardi, María Gabriela; Pasqualini, Christiane Dosne; Ruggiero, Raul Alejandro A differential effect of pregnancy on the growth of subcutaneous implants of four murine tumors has been observed. Two tumors lacking receptors for progesterone and estrogen [methylcholanthrene-induced fibrosarcoma (MC-C) and spontaneous lymphoid leukemia (LB)] exhibited slow kinetics throughout the course of pregnancy, although inhibition was stronger beyond day 10. On the other hand, one of two tumors bearing receptors for progesterone and estrogen [medroxyprogesterone (MPA)-induced mammary adenocarcinoma (C7HI)] exhibited three phases: up to days 8-10 of gestation the tumor grew faster than in virgins, between days 8-10 and 15 it reached a plateau, and beyond day 15 a sharp reduction in tumor mass was observed. The other tumor [mouse mammary tumor virus (MMTV)-induced mammary carcinoma(T2280)] behaved as a typical pregnancy-dependent tumor (i.e., it grew in pregnant but not in virgin mice, regressed soon after delivery, and reassumed its growth at the middle of a second round of pregnancy). Neither MPA nor estrogen affected MC-C and LB tumor growth. On the other hand, MPA-treated mice enhanced C7HI tumor and reciprocally C7HI tumor-bearing mice treated with estrogen strongly inhibited tumor growth. As for T2280, neither MPA nor estrogen alone could promote tumor growth and, in consequence, no tumor developed. However, when MPA plus estrogen was administered in a schedule simulating the successive appearance of these hormones in pregnancy, T2280 grew even faster than in pregnant mice. When the four tumors were implanted in mice bearing grafts of embryonal tissues (teratomas), all of them were inhibited. This antitumor effect was similar to that observed in pregnancy when tumors unresponsive to progesterone and estrogen were tested. On the other hand, with tumors bearing progesterone and estrogen receptors, differences in tumor growth were detected in pregnant and teratoma-bearing mice. This suggested the existence during pregnancy of two factors potentially acting on tumor growth. First, a progesterone and estrogen-mediated hormonal component, which would exert either inhibitory or stimulatory effects only evidenced with tumors bearing hormonal receptors. Secondly, an antitumor effect proportional to the growing embryonal mass, inhibiting all tumors independently of their origin or hormone responsiveness. This antitumor effect could be attributed to a beat-resistant serum factor (1,000-1,200 Da molecular weight) presumably associated with the pathway of the arachidonic acid metabolism. The interplay between the hormonal component and the serum factor associated with embryonal mass could account for some of the largely heterogeneous and otherwise unexplained effects of pregnancy on tumor growth reported in the literature and illustrated by the four tumors studied here.

Linear Viscoelastic Relaxation Modulus of Polydisperse Poly(dimethylsiloxane) Melts Containing Unentangled Chains Deiber, Julio Alcides; Peirotti, Marta Beatriz; Villar, Marcelo Armando; Ressia, Jorge Aníbal; Valles, Enrique Marcelo This work analyzes the relationship between the shear relaxation modulus of entangled, linear and flexible homopolymer blends and its molecular weight distribution (MWD) when a fraction of the sample contains chains with molecular weight M lower than the effective critical molecular weight between entanglements Mceff. This effective critical parameter is defined in terms of the critical molecular weight between entanglements Mc of the bulk polymer that forms the physical network and the effective mass fraction Wceff of the unentangled chains. In the terminal zone of the linear viscoelastic response, the double reptation mixing rule for blended entangled chains and a modified law for the relaxation time of chains in a polydisperse matrix are considered, where the effect of chains with MMceff. This mechanism is independent from the MWD. The unentangled chains with M

Un ejercicio de historia regional comparada. Coacción y mercado de trabajo. Tucumán y Mendoza en el horizonte latinoamericano (segunda mitad del siglo XIX Campi, Daniel Enrique A.; Jorba, Rodolfo Richard El artículo plantea un enfoque comparativo de los procesos de constitución de los mercados laborales en Tucumán y Mendoza en las décadas finales del siglo XIX. El análisis se enmarca en el debate que sobre el peonaje por deudas y –en general sobre las diversas modalidades del trabajo coactivo– se dio en la historiografía latinoamericana, que se complementa con un repaso de las diversas interpretaciones que la cuestión deparó en la historiografía rioplatense y en la historiografía tucumana. En el tratamiento del problema se pone énfasis en las prácticas –en las iniciativas del Estado provincial, de los tomadores de mano de obra asalariada, de los trabajadores–, para concluir proponiendo una hipótesis sobre las razones de la extinción del sistema coactivo. En la misma se contemplan diversas variables, entre las cuales se destacan las estrategias de resistencia de los trabajadores y la aparición en el escenario de un nuevo y decisivo actor, el Estado nacional.

Two new phoxocephalids (Crustacea: Amphipoda: Phoxocephalidae) from the south-west Atlantic Alonso, Gloria Maria A new genus and species, Parafoxiphalus longicarpus, and a new species, Metharpinia protuberantis belonging to the family Phoxocephalidae, are described from the Argentine Sea. Both taxa were collected from sandy substrata in the intertidal and subtidal regions. Parafoxiphalus longicarpus is related to Foxiphalus Barnard, 1979 and they share many characters, most of them in plesiomorphic condition. Metharpinia protuberantis is an intermediate species between Metharpinia Schellenberg, 1931 and Microphoxus Barnard, 1960, and its assignation to the first genus is discussed.

Andean Cenozoic foreland basins: a review Gonzalez Bonorino, Gustavo; Kraemer, P.; Re, G. Interaction between the South American craton and the oceanic Nazca Plate throughout the Cenozoic gave rise to the Andean orogen and a related assemblage of retroarc foreland basins. This review partly summarizes abundant existing data on the sedimentary and tectonic evolution of Andean Cenozoic foreland basins and intends to serve as a geographic and geologic framework for the detailed papers in this special issue. Several of these papers were presented orally at a special Andean session held during the 14th Congreso Geológico Argentino, Salta, Argentina, on September 1999.

Characterization of static bifurcations in the frequency domain Itovich, Griselda Rut; Moiola, Jorge Luis In this paper two-dimensional systems with static bifurcations are considered. An analysis of the bifurcation behavior is proposed using a frequency domain approach. The analyzed bifurcations are known as elementary since they are the building blocks to understand other more complex singularities.

Chaos prediction and bifurcation analysis in control engineering Alonso, Diego; Calandrini, Guillermo Luis; Berns, Daniel Walther; Paolini, Eduardo Emilio; Moiola, Jorge Luis In this paper, two different methods to compute the period-doubling route to chaos (or Feigenbaum chaos) in nonlinear systems are presented. The first one is a semi-analytical procedure, based on a symbolic calculation of an approximate monodromy matrix. The second one takes advantage of software packages for continuation of periodic solutions. Both procedures are used to analyze Chua´s circuit. The second method is also applied to the Rössler system and one of the chaotic systems of Sprott. In all three cases, several period-doubling bifurcation points in the parameter space are detected, allowing to compute a sequence of values supposedly converging to Feigenbaum´s constant. This "experimental´´ computer verification agrees with experiments performed by other researchers in real systems. This material has been used in final projects in a graduate course in dynamical systems.