Scaling laws and dynamics of hashtags on Twitter.

In this paper, we quantify the statistical properties and dynamics of the frequency of hashtag use on Twitter. Hashtags are special words used in social media to attract attention and to organize content. Looking at the collection of all hashtags used in a period of time, we identify the scaling laws underpinning the hashtag frequency distribution (Zipf's law), the number of unique hashtags as a function of sample size (Heaps' law), and the fluctuations around expected values (Taylor's law). While these scaling laws appear to be universal, in the sense that similar exponents are observed irrespective of when the sample is gathered, the volume and the nature of the hashtags depend strongly on time, with the appearance of bursts at the minute scale, fat-tailed noise, and long-range correlations. We quantify this dynamics by computing the Jensen-Shannon divergence between hashtag distributions obtained τ times apart and we find that the speed of change decays roughly as 1/τ. Our findings are based on the analysis of 3.5×109 hashtags used between 2015 and 2016.

Characterizing the Impact of Women in Academic IR: A 12-Year Analysis.

PURPOSE: To determine the representation of female interventional radiology (IR) investigators and elucidate possible gender-specific disparities. MATERIALS AND METHODS: We analyzed 4,884 original, peer-reviewed articles from 2006-2017 in the Journal of Vascular and Interventional Radiology and CardioVascular and Interventional Radiology. Data abstraction and statistical analysis were performed for first and senior author gender, citations, and grants. RESULTS: We found that 84% of first authors and 91.4% of senior authors were male (P < .01). No significant difference was observed versus expected in terms of author gender collaboration combinations (P = 1.00). Each year reflected a 0.3%-0.4% increase in articles published by women (first author: B-value: 0.3, P = .05; senior author: B-value: 0.4, P = .01). No difference was observed in citations or grants received between genders. Female authors received increasing citations and grants each year (citations: first author: B-value: 0.24, P = .05; senior author: B-value: 0.16, P = .15; grants: B-value: 0.88, P = .02). CONCLUSIONS: Women are equally as productive as men as determined by metrics such as number of publications, citations, and grants and are proportionally represented in the literature. No data indicating collaborative or citation/grant discrimination were observed, suggesting that the academic IR community is inclusive of its female constituents and equally respects their research contributions. Based on the statistically significant increases in female authorship observed in this 12-year study, this article reports encouraging trends for the future of women in interventional radiology.

Gender Disparities in Academic Radiology Authorship: A 13-Year Review.

RATIONALE AND OBJECTIVES: Underrepresentation of women in many scientific fields has been linked to biases against female-authored publications in peer-reviewed journals, thereby reducing their opportunities for career development and promotions. The objective of this study is to determine the representation of female academic radiologists in four flagship general radiology journals to elucidate gender-specific trends and disparities in academic radiology. MATERIALS AND METHODS: We analyzed 23,741 peer-reviewed articles published in Radiology, American Journal of Roentgenology, Journal of the American College of Radiology and Academic Radiology from 2007 to 2020. Data abstraction and statistical analysis were performed for author gender, first and last authorship, research funding, and number of citations and grants. Baseline demographics data of academic radiologists was obtained from the Association of American Medical Colleges (AAMC). RESULTS: A total of 72.4% of authors were male with 54% of articles were written by a male first author and male last author. When compared with assumed random pairing, there were significantly fewer Male/Female author combinations and more Female/Female author combinations than expected (p<0.01). Over the 13-year time period, the rate of increase in the number of female authors exceeded the rate of increase in women in academic radiology as reported by the AAMC. Female last authors received on average 3.2 less citations than their male counterparts (p=.03). Of manuscripts with last female authors, 31.7% of female last authors were funded compared to 25.9% of last male authors. CONCLUSION: This study showed the increasing numbers and higher productivity of female authors in academic radiology, suggesting progress is being made in overcoming gender disparities.

In-flight and collisional dissipation as a mechanism to suppress Fermi acceleration in a breathing Lorentz gas.

Some dynamical properties for a time dependent Lorentz gas considering both the dissipative and non dissipative dynamics are studied. The model is described by using a four-dimensional nonlinear mapping. For the conservative dynamics, scaling laws are obtained for the behavior of the average velocity for an ensemble of non interacting particles and the unlimited energy growth is confirmed. For the dissipative case, four different kinds of damping forces are considered namely: (i) restitution coefficient which makes the particle experiences a loss of energy upon collisions; and in-flight dissipation given by (ii) F=-ηV(2); (iii) F=-ηV(µ) with µ≠1 and µ≠2 and; (iv) F=-ηV, where η is the dissipation parameter. Extensive numerical simulations were made and our results confirm that the unlimited energy growth, observed for the conservative dynamics, is suppressed for the dissipative case. The behaviour of the average velocity is described using scaling arguments and classes of universalities are defined.

Shrimp-shape domains in a dissipative kicked rotator.

Some dynamical properties for a dissipative kicked rotator are studied. Our results show that when dissipation is taken into account a drastic change happens in the structure of the phase space in the sense that the mixed structure is modified and attracting fixed points and chaotic attractors are observed. A detailed numerical investigation in a two-dimensional parameter space based on the behavior of the Lyapunov exponent is considered. Our results show the existence of infinite self-similar shrimp-shaped structures corresponding to periodic attractors, embedded in a large region corresponding to the chaotic regime.

Fermi acceleration and scaling properties of a time dependent oval billiard.

We consider the phenomenon of Fermi acceleration for a classical particle inside an area with a closed boundary of oval shape. The boundary is considered to be periodically time varying and collisions of the particle with the boundary are assumed to be elastic. It is shown that the breathing geometry causes the particle to experience Fermi acceleration with a growing exponent rather smaller as compared to the no breathing case. Some dynamical properties of the particle's velocity are discussed in the framework of scaling analysis.

Thermodynamics of a time-dependent and dissipative oval billiard: A heat transfer and billiard approach.

We study some statistical properties for the behavior of the average squared velocity-hence the temperature-for an ensemble of classical particles moving in a billiard whose boundary is time dependent. We assume the collisions of the particles with the boundary of the billiard are inelastic, leading the average squared velocity to reach a steady-state dynamics for large enough time. The description of the stationary state is made by using two different approaches: (i) heat transfer motivated by the Fourier law and (ii) billiard dynamics using either numerical simulations and theoretical description.

In-flight dissipation as a mechanism to suppress Fermi acceleration.

Some dynamical properties of time-dependent driven elliptical-shaped billiards are studied. It was shown that for conservative time-dependent dynamics the model exhibits Fermi acceleration [Phys. Rev. Lett. 100, 014103 (2008).] On the other hand, it was observed that damping coefficients upon collisions suppress such a phenomenon [Phys. Rev. Lett. 104, 224101 (2010)]. Here, we consider a dissipative model under the presence of in-flight dissipation due to a drag force which is assumed to be proportional to the square of the velocity of the particle. Our results reinforce that dissipation leads to a phase transition from unlimited to limited energy growth. The behavior of the average velocity is described using scaling arguments.