## 1 Introduction  
Since the 1970s, with the development of integrated circuit technology and semiconductor components, there has been a growing interest in designing nonlinear circuits and systems and the phenomena accompanying them [1, 2]. The paper [1] defined the term nonlinear technology and indicated that it includes nonlinear devices, as well as numerical methods and strategies. Nonlinear technology, especially developed in the 21 st Century, aims to design new devices and equipment that can achieve added values over those designed according to linear strategies [1]. In [1], several aspects of nonlinear technology were discussed regarding selected papers published in the journal _Energies_ over the past two years. Chaos theory deals with nonlinear systems that exhibit complex, seemingly random behavior. The theory is an interdisciplinary area of research and has significance in many fields of science. High sensitivity to initial conditions, emergent behavior, and bifurcations are peculiar features found in nonlinear systems [2]. The paper [2] pointed out the steadily increasing trend of papers related to chaos over the past few years and discussed 21 papers in this field from the past two years that the authors of the Editorial considered particularly relevant to the field. The first task in simulating the behavior of a nonlinear electronic circuit is to find the DC operating point. The simulation is important because the operating point is required for other circuit analyses. For example, the DC operating point is used as a starting point for transient analysis and to determine the parameters of small-signal models in AC analysis. When performing DC analysis of dynamic nonlinear circuits, all capacitors and inductors are removed from the original system. The inductor terminals are shorted, and the capacitor terminals are left open [3; 4]. Electronic analog circuits contain nonlinear elements such as diodes and transistors. These semiconductor devices are described by many nonlinear equations and are characterized by many parameters. The newer the technology, the more complex the description of a single device. Therefore, the general circuit description leads to a large number of nonlinear equations. The robust and efficient calculation of the solutions to a set of nonlinear equations is a theoretically and practically difficult problem. The calculation of the DC operating points of a nonlinear circuit and the so-called power flow solutions of a power grid are among the most-important tasks in electronic and electrical engineering. Certain circuits, such as amplifiers and logic gates, are designed to have a unique DC operating point. However, circuits such as flip-flops, static shift registers, static random access memory (RAM) cells, latch circuits, line receivers, Hopfield neurons, negative resistance circuits, and Schmitt triggers possess multiple isolated DC operating points [5; 6]. There are also many important circuits containing the above circuits as sub-circuits [6]. Furthermore, transconductance-amplifier-based filters (\(G_{m}\)-C filters) and most log-domain filters use negative differential resistances produced by positive feedback to increase the quality factor of the integrator, which can lead to the appearance of an undesirable operating point (or points) and make the circuit unusable [7]. Even simple circuits fabricated in MOS/CMOS technology can have multiple solutions. The paper [8] presented a two-transistor MOS circuit with five operating points. An open-loop analysis showed that simple tristable latches could have five operating points [9]. Multiple operating points sometimes result in circuits such that the systems do not operate as designed. In such a case, verifying the presence of multiple DC solutions before the production of the circuits is essential [10]. Determining the solution at equilibrium is also an important issue when studying chaos. Finding multiple DC solutions of nonlinear resistive circuits is a difficult task.