The concept of function is a threshold concept in mathematics since it is essential for a deep understanding of the subject and is often problematic for students. Despite the importance of this concept, numerous studies have found that students have several difficulties and misconceptions about it. This work aims to assess, classify and synthesize the existing information about the learning difficulties and misconceptions related to the concept of function. In this sense, the main achievement of the work is the synthesis and classification by topics of the information gathered from the literature.

The widespread deployment of LEDs for illumination purposes has open the door to the use of these devices for visible light communications (VLC). Most lighting fixtures are mounted with phosphor-based white LEDs, and a driver connected to the LED is also required for VLC. This paper shows that the parasitic effects introduced by this setup change the frequency response of the intrinsic LED. A linear model to characterize the whole setup is proposed, as well as a methodology to extract its parameters. This methodology allows the designer to characterize the frequency response of LEDs without the additional difficulty of knowing the specific parasitic components introduced by the setup. The proposed model offers an accurate estimation of the slope of the LED frequency response in order to broaden the frequency range in which the model is useful to characterize and simulate VLC links. This was corroborated with the characterization of three commercial white LEDs whose measured and modeled frequency responses matched perfectly.

A high-speed non-binary LDPC decoder based on Trellis Min-Max algorithm with layered schedule is presented. The proposed approach compresses the check-node output messages into a reduced set, decreasing the number of messages sent to the variable node. Additionally, the memory resources from the layered architecture are reduced. The proposed decoder was implemented for the (2304,2048) NB-LDPC code over GF(16) on a Virtex-7 FPGA and in a 90 nm CMOS process. Our implementation outperforms state-of-the-art NB-LDPC decoder implementations for both technologies, achieving a throughput of 630 and 965 Mbps, respectively.

LED lamps have become the predominant technology for lighting thanks to their low power consumption. Recently, the joint use of these devices for lighting and data transmission has aroused interest in the research community as a future green communication technology. During this time, many of the advances developed in this area have been demonstrated with experimental setups using LED drivers based on bias-T, thanks to their ease to combine signal modulation and LED bias. This article discusses the impact of the output impedance of the bias-T driver and the driver-LED connection parasitics on the performance and energy efficiency of the VLC system. It is concluded that the output impedance of the driver not only affects energy efficiency but also modifies the frequency response and, therefore, the LED's capacity to be modulated. A low output impedance driver is shown to offer a substantial improvement in the energy efficiency of the VLC driver.

Pregraduate students often have low success expectations toward their thermodynamics courses, which are often considered too abstract and remarkably difficult to understand. For this reason, they may not even try to reach any level of comprehension while settling for reproducing mathematical calculations and memorizing definitions to pass the exams. Traditional lectures on thermodynamics, focusing on mathematical deductions while neglecting the qualitative characterization of the concepts behind the equations, do not help in this respect. Aiming at a change in the teaching practice and focused on the second law of thermodynamics, the main goals of this work are to characterize the way of reasoning of the expert; to present a review on the most important learning difficulties encountered by students and categorize them into three groups: the disregard of qualitative understanding, the inherent conceptual difficulties, and those related to the students’ previous knowledge; and to propose some suitable teaching practices to assist instructors in this difficult but rewarding task.