We investigate high-order above-threshold ionization (HATI) of krypton atoms by a bicircular laser field, which consists of two coplanar co- or counter-rotating circularly polarized fields of frequencies rw and sw. We show that the photoelectron spectra in the HATI process, presented in the momentum plane, exhibit the same discrete rotational symmetry as the driving field. We also analyze HATI spectra for various combinations of the intensities of two field components for co- and counter-rotating fields. We find that the appearance of high-energy plateau for the counter-rotating case is vary sensitive to the laser intensity ratio, while the plateau is always absent for the co-rotating bicircular field.

By using the KAM(Kolmogorov-Arnold-Moser) theory and time reversal symmetries, we investigate the stability of the equilibrium solutions of the system: x n + 1 = 1 y n , y n + 1 = β x n 1 + y n , n = 0 , 1 , 2 , … , where the parameter β > 0 , and initial conditions x 0 and y 0 are positive numbers. We obtain the Birkhoff normal form for this system and prove the existence of periodic points with arbitrarily large periods in every neighborhood of the unique positive equilibrium. We use invariants to find a Lyapunov function and Morse’s lemma to prove closedness of invariants. We also use the time reversal symmetry method to effectively find some feasible periods and the corresponding periodic orbits.

INTRODUCTION Occupational diseases are diseases caused by occupational exposures at the workplace, while diseases related to work are diseases caused by many factors, wherein the harmful work conditions are one of the possible causes. Dental medicine is a profession with a high risk of developing occupational diseases. The aim of this paper was to present the results of a survey about occupational health risks and health related habits among dental students at the School of Dental Medicine University of Zagreb and to show how this survey led to an improvement in education by introducing a new course into the dental curriculum. PARTICIPANTS AND METHODS Students of all years at the School of Dental Medicine University of Zagreb (total of 663) were offered to participate in a survey about occupational health risks and health related habits. RESULTS A questionnaire was completed by 351 students. 28.0% of male students and 22.5% of female students were smokers. During the first two years of study, up to 84.6% of students consumed alcoholic beverages at least once a week. Prior to enrollment in the university, 85.3% male and 77.6% female students were engaged in sports. The significant drop in the number of students who participated in sports was noticed in the first study year in both sexes. Student awareness of the health risks related to dental profession increases with the year of study. Most students believe that occupational diseases can be prevented. CONCLUSION The results have shown that students enter the School of Dental Medicine with a relatively low level of awareness of the health hazards of dental profession. Although the level of awareness increases with years of study, harmful habits such as smoking, alcohol consumption, and the lack of physical activity also remain present in the later years of study.