Maximal Number of Successors in a Nginar(1) Process

2000 Mathematics Subject Classification: 60J80, 60J20, 60J10, 60G10, 60G70, 60F99.In this paper we obtain upper and lower bounds for normalized sequences of maxima, associated with a stationary integer-valued autoregressive process of first order with geometricmarginals (NGINAR(1) process). These processes are a special case of AR(1) processes and strictly stationary ergodic Markov chains.The first author is partially supported by grant RD–05-333/2010 (RD-07-1028) of Shumen University, Bulgaria

Mechanochemical synthesis and characterization of nanodimensional iron–cobalt spinel oxides

Iron–cobalt spinel oxide nanoparticles, CoxFe3−xO4 (x = 1, 2), of sizes below 10 nm have been prepared by combining chemical precipitation with high-energy ball milling. For comparison, their analogues obtained by thermal synthesis have also been studied. The phase composition and structural properties of the obtained materials have been investigated by means of X-ray diffraction, Mössbauer spectroscopy, infrared spectroscopy, temperature-programmed reduction and magnetization measurements. X-ray diffraction shows that after 1 h of mechanical treatment ferrites are formed. The measurement techniques employed indicate that longer milling induces an increase in crystal size while crystal defects decrease with treatment time. Magnetization and reduction properties are affected by the particles size, the iron/cobalt ratio and the synthesis conditions

Time series analysis for financial market meltdowns

There appears to be a consensus that the recent instability in global financial markets may be attributable in part to the failure of financial modeling. More specifically, current risk models have failed to properly assess the risks associated with large adverse stock price behavior. In this paper, we first discuss the limitations of classical time series models for forecasting financial market meltdowns. Then we set forth a framework capable of forecasting both extreme events and highly volatile markets. Based on the empirical evidence presented in this paper, our framework offers an improvement over prevailing models for evaluating stock market risk exposure during distressed market periods. --ARMA-GARCH model,»-stable distribution,tempered stable distribution,value-at-risk (VaR),average value-at-risk (AVaR)

Detection of genital mycoplasmas in men attending medical clinics in Sofia, Bulgaria

PURPOSE: The role of Ureaplasma urealyticum (U. urealyticum), Mycoplasma hominis (M. hominis) and Mycoplasma genitalium (M. genitalium) in male genital disorders deserves a more intensive research. The aim of this study was to determine the prevalence rate of these pathogens in symptomatic and asymptomatic sexually active males in Bulgaria.MATERIAL AND METHODS: A total of 260 symptomatic and asymptomatic men were included in the study. Their urethral swabs were examined for the presence of the mentioned microorganisms by using polymerase chain reaction (PCR).RESULTS: The prevalence rates of these pathogens were the following: U. urealyticum - 4.6%; M. hominis - 0.8%, and M. genitalium - 3.1%. No coinfections were registered at all.CONCLUSION: The present investigation of these mycoplasmas is the first attempt to provide certain epidemiological information about these infections in Bulgaria. Their continuous monitoring is of practical importance for public health.Scripta Scientifica Medica 2013; 45(4): 46-49

MICROBIOLOGICAL CHARACTERISATION OF Haemophilus influenzae STRAINS ISOLATED FROM PATIENTS WITH INVASIVE AND RESPIRATORY DISEASES

A total of 175 H. influenzae strains were collected between 1994 and 2009 from all aged patient groups. The strains were isolated from patients with invasive and community-acquired respiratory tract infections. All strains were identified according to standard microbiological methods. Serotyping was done by a coagglutination test and by molecular PCR capsular genotyping. Beta-lactamase production was determined by the chromogenic cephalosporin test with nitrocephin as substrate. Most of the isolated H. influenzae strains were from children under 5 years of age (57.7%). Overall, 61 strains belonged to serotype b (34.9%) by the means of PCR capsular typing, 1 strain was type f, and 113 isolates (64.6%) were non-typeable (non-encapsulated) H. influenzae. Among the infants and children with meningitis or other invasive infections, aged 2 month to 5 years, all strains, except one, were serotype b. In respiratory tract infections (pneumonia, otitis media, sinusitis and people with chronic pulmonary diseases - exacerbations of COPD, bronchiectasis, cystic fibrosis) the most common - 96.5% were non-typeable strains in both groups children and adults. Overall, the prevalence of beta-lactamase production was 19.4%. But, it was much higher for invasive strains from CSF isolates - 37.7%, 25% in blood samples, and 37.5% in otitis media causative strains. Beta-lactamase production was less frequent in respiratory tract isolates - in sputum 13.3% and in URT samples - 2.3%. The rate of beta-lactamase production in CSF isolates has not changed for the last 10 years.PCR capsular genotyping method has to be performed for all non-b-type strains. The implementation of Hib vaccine in our country will be accompanied by a reduction in invasive diseases caused by H. influenzae type b in children, but it is not useful in preventing infections caused by non-typeable H. influenzae strains

Synthesis of Improved Catalytic Materials for High-Temperature Water-gas Shift Reaction

In this investigation, we report the preparation and characterization of Co-, Cu- and Mn-substituted iron oxide catalytic materials supported on activated carbon. Co-precipitation method and low temperature treatment were used for their synthesis. The influence of chemical composition, stoichiometry, particle size and dispersity on their catalytic activity was studied. Samples were characterized in all stages of their co-precipitation, heating and spend samples after catalytic tests. The obtained results from room and low temperature Mössbauer spectroscopy were combined with analysis of powder X-ray diffraction patterns (XRD). They revealed the preparation of nano-sized iron oxide materials supported on activated carbon. Relaxation phenomena were registered also for the supported phases. The catalytic performance in the water-gas shift reaction was studied. The activity order was as follows: Cu0.5Fe2.5O4 > Co0.5Fe2.5O4 > Mn0.5Fe2.5O4. Catalytic tests demonstrated very promising results and potential application of studied samples due to their cost-effective composition

Spark plasma sintering synthesis of Ni1−xZnxFe2O4 ferrites: Mössbauer and catalytic study

Nickel-zinc ferrite nanoparticles, Ni1-xZnxFe2O4 (x ¼ 0, 0.2, 0.5, 0.8, 1.0) were prepared by combination of chemical precipitation and spark plasma sintering (SPS) techniques and conventional thermal treatment of the obtained precursors. The phase composition and structural properties of the obtained materials were investigated by X-ray diffraction and Mössbauer spectroscopy and their catalytic activity in methanol decomposition was tested. A strong effect of reaction medium leading to the transformation of ferrites to a complex mixture of different iron containing phases was detected. A tendency of formation of Fe-carbide was found for the samples synthesized by SPS, while predominantly iron-nickel alloys ware registered in TS obtained samples. The catalytic activity and selectivity in methanol decomposition to CO and methane depended on the current phase composition of the obtained ferrites, which was formed by the influence of the reaction medium

Prevalence of Helicobacter pylori is still high among symptomatic Bulgarian children

Helicobacter pylori positivity was assessed among 656 symptomatic children in 2010–2017. Overall infection prevalence was 24.5% and a significantly higher rate was detected in girls (28.5%) compared to boys (20.0%). Moreover, in children with duodenal ulcer, H. pylori prevalence was higher (47.4%) compared with the rest (23.9%). On the contrary, the infection was detected 1.9-fold less frequently in patients with GERD (14.5%) compared with the other (27.0%) patients and 2.1-fold less often in the presence of duodenogastric reflux (bile) reflux (13.0%) compared with the absence of the reflux (27.0%). No significant difference was observed between the younger (aged ≤7 years, 20.0%) and the older (aged 8–18 years, 25.5%) patients. H. pylori infection rate in Bulgarian pediatric patients between 2010 and 2017 was 2.5-fold lower than that in 1996–2006. In conclusion, H. pylori infection is still an important concern for Bulgarian children, although having decreased by about 1.8%/yearly over 21 years. This study reveals the importance of H. pylori diagnostics even in the youngest symptomatic children and demonstrates an inverse association between either GERD or bile reflux and H. pylori infection