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Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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14 pages, 3687 KiB

Open AccessEditor’s ChoiceArticle

Twist and Glide Symmetries for Helix Antenna Design and Miniaturization

by Ángel Palomares-Caballero, Pablo Padilla, Antonio Alex-Amor, Juan Valenzuela-Valdés and Oscar Quevedo-Teruel

Symmetry 2019, 11(3), 349; https://doi.org/10.3390/sym11030349 - 8 Mar 2019

Cited by 18 | Viewed by 8760

Abstract

Here we propose the use of twist and glide symmetries to increase the equivalent refractive index in a helical guiding structure. Twist- and glide-symmetrical distributions are created with corrugations placed at both sides of a helical strip. Combined twist-and glide-symmetrical helical unit cells are studied in terms of their constituent parameters. The increase of the propagation constant is mainly controlled by the length of the corrugations. In our proposed helix antenna, twist and glide symmetry cells are used to reduce significantly the operational frequency compared with conventional helix antenna. Equivalently, for a given frequency of operation, the dimensions of helix are reduced with the use of higher symmetries. The theoretical results obtained for our proposed helical structure based on higher symmetries show a reduction of 42.2% in the antenna size maintaining a similar antenna performance when compared to conventional helix antennas. Full article

(This article belongs to the Special Issue Higher Symmetries and Its Application in Microwave Technology, Antennas and Metamaterials)

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25 pages, 1302 KiB

Open AccessEditor’s ChoiceArticle

Biological Bases of Beauty Revisited: The Effect of Symmetry, Averageness, and Sexual Dimorphism on Female Facial Attractiveness

by Alex L. Jones and Bastian Jaeger

Symmetry 2019, 11(2), 279; https://doi.org/10.3390/sym11020279 - 21 Feb 2019

Cited by 84 | Viewed by 26264

Abstract

The factors influencing human female facial attractiveness—symmetry, averageness, and sexual dimorphism—have been extensively studied. However, recent studies, using improved methodologies, have called into question their evolutionary utility and links with life history. The current studies use a range of approaches to quantify how important these factors actually are in perceiving attractiveness, through the use of novel statistical analyses and by addressing methodological weaknesses in the literature. Study One examines how manipulations of symmetry, averageness, femininity, and masculinity affect attractiveness using a two-alternative forced choice task, finding that increased masculinity and also femininity decrease attractiveness, compared to unmanipulated faces. Symmetry and averageness yielded a small and large effect, respectively. Study Two utilises a naturalistic ratings paradigm, finding similar effects of averageness and masculinity as Study One but no effects of symmetry and femininity on attractiveness. Study Three applies geometric face measurements of the factors and a random forest machine learning algorithm to predict perceived attractiveness, finding that shape averageness, dimorphism, and skin texture symmetry are useful features capable of relatively accurate predictions, while shape symmetry is uninformative. However, the factors do not explain as much variance in attractiveness as the literature suggests. The implications for future research on attractiveness are discussed. Full article

(This article belongs to the Special Issue Fluctuating asymmetry: A predictor of human life history outcomes)

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13 pages, 355 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Nonclassical Symmetry Solutions for Non-Autonomous Reaction-Diffusion Equations

by Bronwyn H. Bradshaw-Hajek

Symmetry 2019, 11(2), 208; https://doi.org/10.3390/sym11020208 - 12 Feb 2019

Cited by 12 | Viewed by 3761

Abstract

The behaviour of many systems in chemistry, combustion and biology can be described using nonlinear reaction diffusion equations. Here, we use nonclassical symmetry techniques to analyse a class of nonlinear reaction diffusion equations, where both the diffusion coefficient and the coefficient of the reaction term are spatially dependent. We construct new exact group invariant solutions for several forms of the spatial dependence, and the relevance of some of the solutions to population dynamics modelling is discussed. Full article

(This article belongs to the Special Issue Lie Symmetries at Work in Biology and Medicine)

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34 pages, 2916 KiB

Open AccessEditor’s ChoiceArticle

Analysis of the Digital Divide Using Fuzzy Forecasting

by Paweł Ziemba and Jarosław Becker

Symmetry 2019, 11(2), 166; https://doi.org/10.3390/sym11020166 - 1 Feb 2019

Cited by 37 | Viewed by 5581

Abstract

In the countries of the former Eastern Bloc (Central and Eastern Europe) belonging to the European Union, a gradual elimination of the technological gap, greater expenditures, competitiveness and productivity can be noticed. In this context, analysis and forecast of the level of accessibility and use of ICT (information and communication technologies) by households in these countries has become interesting. It allows for the selection of digitally excluded regions, or those threatened with this phenomenon in the coming years (2018–2020). To carry out the analysis, a framework based on fuzzy numbers and the NEAT F-PROMETHEE (New Easy Approach To Fuzzy-PROMETHEE) method was developed. The potential of the fuzzy outranking approach taking into account the uncertainty of input data (criteria and preferences) has been demonstrated as an alternative to the IDI (ICT Development Index) methodology widely used in research on regional ICT development based on composite indices. Research has shown that Estonia is the leader in the area of ICT expansion among households, and in the next three years will definitely maintain its dominant position. Slovenia follows shortly after, followed by Latvia. At the end of the ranking were the countries with the largest percentage in Central and Eastern Europe of population excluded digitally or threatened with this phenomenon; these are: Poland, Slovakia, Bulgaria and, in last place, Romania. Within this framework, the robustness of the obtained rankings to change in the degree of uncertainty of preferences was also examined. It turned out that eliminating the uncertainty of preferences resulted in an increase in the uncertainty of the aggregate, fuzzy grades obtained at the output. Full article

(This article belongs to the Special Issue Multi-Criteria Decision Aid methods in fuzzy decision problems)

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18 pages, 391 KiB

Open AccessEditor’s ChoiceArticle

Spacetime Symmetry and LemaîTre Class Dark Energy Models

by Irina Dymnikova and Anna Dobosz

Symmetry 2019, 11(1), 90; https://doi.org/10.3390/sym11010090 - 15 Jan 2019

Cited by 7 | Viewed by 4032

Abstract

We present the regular cosmological models of the Lemaître class with time-dependent and spatially inhomogeneous vacuum dark energy, which describe relaxation of the cosmological constant from its value powering inflation to the final non-zero value responsible for the present acceleration in the frame of one self-consistent theoretical scheme based on the algebraic classification of stress-energy tensors and spacetime symmetry directly related to their structure. Cosmological evolution starts with the nonsingular non-simultaneous de Sitter bang, followed by the Kasner-type anisotropic expansion, and goes towards the present de Sitter state. Spacetime symmetry provides a mechanism of reducing cosmological constant to a certain non-zero value involving the holographic principle which singles out the special class of the Lemaître dark energy models with the global structure of the de Sitter spacetime. For this class cosmological evolution is guided by quantum evaporation of the cosmological horizon whose dynamics entirely determines the final value of the cosmological constant. For the choice of the density profile modeling vacuum polarization in a spherical gravitational field and the GUT scale for the inflationary value of cosmological constant, its final value agrees with that given by observations. Anisotropy grows quickly at the postinflationary stage, then remains constant and decreases to

A < 10^{- 6}

when the vacuum density starts to dominate. Full article

(This article belongs to the Special Issue Cosmological Inflation, Dark Matter and Dark Energy)

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51 pages, 737 KiB

Open AccessEditor’s ChoiceArticle

Vacuum Condensate Picture of Quantum Gravity

by Herbert W. Hamber

Symmetry 2019, 11(1), 87; https://doi.org/10.3390/sym11010087 - 14 Jan 2019

Cited by 13 | Viewed by 4506

Abstract

In quantum gravity perturbation theory in Newton’s constant G is known to be badly divergent, and as a result not very useful. Nevertheless, some of the most interesting phenomena in physics are often associated with non-analytic behavior in the coupling constant and the existence of nontrivial quantum condensates. It is therefore possible that pathologies encountered in the case of gravity are more likely the result of inadequate analytical treatment, and not necessarily a reflection of some intrinsic insurmountable problem. The nonperturbative treatment of quantum gravity via the Regge–Wheeler lattice path integral formulation reveals the existence of a new phase involving a nontrivial gravitational vacuum condensate, and a new set of scaling exponents characterizing both the running of G and the long-distance behavior of invariant correlation functions. The appearance of such a gravitational condensate is viewed as analogous to the (equally nonperturbative) gluon and chiral condensates known to describe the physical vacuum of QCD. The resulting quantum theory of gravity is highly constrained, and its physical predictions are found to depend only on one adjustable parameter, a genuinely nonperturbative scale

ξ

in many ways analogous to the scaling violation parameter

Λ_{\bar{M} S}

of QCD. Recent results point to significant deviations from classical gravity on distance scales approaching the effective infrared cutoff set by the observed cosmological constant. Such subtle quantum effects are expected to be initially small on current cosmological scales, but could become detectable in future high precision satellite experiments. Full article

(This article belongs to the Special Issue Symmetry and Quantum Gravity)

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