Indeed, until now,  
Figure 2: Classification of the 90 contributions related to chaos according to the main topic addressed. the undiscovered and unexplored aspects of investigating complex systems represent an appealing research topic. The classic Duffing forced oscillator, like the Lorenz system, represents one of the most studied paradigms for chaos and other complex behavior. The paper [6], by [PERSON] et al., has been selected even though the title does not contain the word chaos since this type of oscillator is a cornerstone in the chaos theory of forced systems. The authors show that the studied system, consisting of a vertical rotor, presents nonlinear jump resonance. An accurate numerical experiment has been reported, and the study includes a detailed analytical section. The paper is also included in the Editorial because jump resonance can be considered as a precursor of chaos in forced dynamical systems. The core of synchronization, and, therefore, the key of actively using chaos, is the design of the observer system. Taking into account a master-slave coupling configuration, the slave is an appropriate nonlinear observer of the master. The authors of [7] introduce an adaptive observe scheme for synchronizing two chaotic systems. This is based on the interaction between the observer and an adaptive controller. The suitable observer and the adaptive controller work on transformed systems. The convergence of the observer is proven, and an accurate analytical formulation of the problem is presented. The numerical examples are referred to the chaotic Arneodo system. The theoretical aspects are discussed in detail and with appealing practical results. The analysis of time-series regarding the water level of rivers is the subject of the contribution [8] by [PERSON] et al. Indeed, chaotic, and therefore in some sense deterministic, oscillations in the river level can open the way to control actions. The authors use the return map approach in order to analyse the time-series. Moreover, the study is also addressed to characterize the attractors of the system and to evaluate all the permutations that allow them to characterize the series. The conclusions positively consider the established chaoticity of the time-series. The contribution opens a discussion on how to use the theory of chaos in evaluating models in hydrogeology and to control river levels. The paper [9] by [PERSON] et al. regards the model of waves within a lattice of simple loss-less devices with a nonlinear load at the end. The paper discusses how waves can be modeled with 1D chaotic maps. The dynamics of the system is accurately analysed for different load parameters. In the contribution, a bifurcation analysis is also reported, as well as an accurate estimation of the Lyapunov exponents. In the paper [10], [PERSON] et al. report an accurate new problem in the area of chaotic vibrations. The discovery of chaos in damping systems allows researchers to control the behavior of the system. The time-series analysis is performed by using more parameters that confirm the chaotic behavior of the experimental system. Moreover, attention has been given to multi-time-scale chaotic characteristics by using the so-called Improved Variational Model Decomposition (IVMD) method. The introduction of chaotic models to study internet dynamics is the subject of the paper [11]. It includes a detailed state-of-the-art and an innovative strategy to use Petri networks in modeling Active Queue Management (AQM) schemes. The analysis was performed by using a two-parameter dynamical map. The numerical results confirm the suitability of the proposed modeling strategy. The image encryption is the main subject of [12]. [PERSON] et al. propose an hyperchaotic system with a single parameter to generate pseudo-random signals. Moreover, an approach based on elliptic curve cryptography is used. In order to validate their approach, the authors present more experiments with encouraging performance indices. The topic of the contribution [13] by [PERSON] et al.