CR4® - The Engineer's Place for News and Discussion®

Abe's Blog

This is an eclectic blog. Not only will I talk about technologies, engineering, nanotechnology, but also about education, music, art, and other human endeavors. After all, humans are not only engineers or doctors.

German Christmas Markets

Posted December 04, 2017 11:15 AM by amichelen

We are about to start the 2017 holiday season, and I would like to talk about it. Since my childhood I have always enjoyed the beauty of these days: the scent of the Christmas tree in the living room of my house, the lights and the music that spreads all over the city, the camaraderie of the people and, of course, the presents.

This year my family and I will be spending the holidays in Germany and the Czech Republic. We will visit Berlin, Dresden, Leipzig and Prague. Germany is one of the countries that celebrates Christmas in an “old-fashioned” way with so many traditions. In particular I love to visit the Christmas markets. In Berlin alone there are over 60 markets ranging from the most traditional to the modern. In Munich, Frankfurt and other big cities there are many as well.

Dresden Striezelmarkt, Germany’s oldest market at the Altmarkt square. Source:

However – maybe the CR4 community can tell me otherwise and correct me – the most beautiful Christmas markets are in Dresden, where we will spend most of our time. There are eleven completely different markets in the city, open from 10 am to 9 pm from November 29 to December 24.

Of these eleven, the market called Striezelmarkt is the oldest in Germany. In 1434, the elector Frederick II authorized the opening of the market to be held on the Altmarkt square. At that time the market was a meat market, where the townspeople could select the roast for their Christmas dinner. This year the Striezelmarkt celebrates its 583rd year of continuous operation in exactly the same location!


Another beautiful and traditional Dresden market takes place in the square of the reconstructed Frauenkirche (church of Our Lady, in English). This church, along with many baroque buildings in the center of Dresden, were destroyed in February of 1945 during the two days of uninterrupted bombing by the Allies at the end of the World War II. For 50 years the ruins of the church were kept piled up for all to see as a war memorial. In 1994, after the reunification of Germany, a world effort started for the reconstruction of the church. In 2005 the final stone was put in place. The Christmas Market in the square of the Frauenkirche is visited by thousands of visitors every year.

The reconstructed Frauenkirche. Source:

Aerial view of the Church of Our Lady. Source: Wikipedia

The market at the Frauenkirche square. Source:

Another city we will visit is Leipzig. Here we will attend the Christmas Oratory of the St. Thomas Boys Choir, a choir created in 1212. The St. Thomas Church is the place where Johann Sebastian Bach spent the last 30 years of his life, where he was Thomas Cantor and where he is buried. I am looking forward to visiting the tomb of the greatest composer ever, and to listen to the boys choir.

Also in Leipzig, we will have dinner and some beers at the Auerbach's Keller (Auerbach’s Cellar, in English), a wine bar and restaurant dating back to 1438, and the place where a young Goethe ate and drank while studying in Leipzig. The Cellar is world famous because in one of the first scenes of Goethe’s play Faust the Cellar is where Mephistopheles takes Faust to make the famous deal.

Mephistopheles bewitching the students, sculptures at the Cellar's entrance. Source: Wikipedia.

Finally we will visit beautiful Prague. There is there also a festive environment during Christmas, including Christmas Markets as well. I will be happy to re-visit the coffee houses where Einstein, Kafka and other great people used to frequent and the Jewish cemetery where Kafka is buried.

Cheers to all! Let us all know about your holidays!

I leave you with this beautiful video about the Striezelmarkt:

29 comments; last comment on 12/12/2017
View/add comments

Las Mejores Universidades de la América Latina

Posted October 19, 2017 4:20 PM by amichelen

Este es mi primer blog en Ingeniería en Español a pesar que lo cree algunos meses atrás. Mi tiempo ha sido limitado para escribir el blog por razones de trabajo y otras cosas. A partir de ahora, sin embargo, prometo escribir con regularidad.

Quiero empezar con un tópico que podría parecer controversial, dado que, para algunos, envuelve orgullo nacional y muchas veces personal.

El tópico: Cuales son las mejores universidades the la America Latina?

Hay varias instituciones que crean rankings de las univerisdades del mundo. Las dos mas importantes, sin embargo, son ambas británicas: Times Higher Education (THE) y el QS World University Ranking. Estas son dos publicaciones de gran prestigio que son “escuchadas” por compañias y gobiernos alrededor del mundo. Cada una cataloga las universidades a nivel mundial y por región.

Asi los índices generados para 2017 por THE y QS indican que hay muy pocos países en la America Latina con universidades que puedan “competir” a nivel mundial. El ranking the THE para 2017 incluye 81 universidades latinoamericans, pero solo 8 paises estan incluidos.

Considerando solameant el ranking de THE, en el grupo de las 50 primeras posiciones estan 18 de Brasil, 15 de Chile, 5 de Colombia y 8 de Mejico, entre otras. Entre las primeras 10 posiciones hay 5 Brasileñas, 2 de Chile, 2 de Méjico y 1 de Colombia.

Estas son las 10 primeras:

1. Universidad Estatal de Campiñas (Brasil)

2. Universidad de Sao Paulo (Brasil)

3. Pontificia Universidad Católica de Chile (Chile)

4. Universidad de Chile (Chile)

5. Universidad de los Andes (Colombia)

6. Instituto Tecnológico y de Estudios Superiores de Monterrey (México)

7. Universidad Federal de Sao Paulo (Brasil)

8. Universidad Federal de Río de Janeiro (Brasil)

9. Pontificia Universidad Católica de Río de Janeiro (Brasil)

10. Universidad Nacional Autónoma de México

Ahora me pregunto, ¿qué pasa con los otros países latinoamericanos? Entre Latino America y el Caribe hay 33 paises; ¿porqué solamente hay 8 en el ranking mundial?

¿Qué les parece? Podriamos enumerar mil justificaciones de porqué la America Latina no ha podido llevar centros superiores de estudio al nivel de otras regiones.

25 comments; last comment on 10/31/2017
View/add comments

Race Tracks Are Not Good For The Horses

Posted August 01, 2017 3:56 PM by amichelen

Now, in the middle of the boreal summer, the sad story of the horses at race tracks like in Saratoga Springs, comes to mind. On average 24 horses die per week in US race tracks, during training, and during the races. The horses are commodities for their owners. If a horse breaks a leg it is killed by the owner, instead of retiring the animal and letting him spend the rest of his days in a pleasant place. To do this, the owner would need to spend money on a horse that is useless for him as a money machine.

This week, at the beautiful Saratoga Springs (New York) race track, two more horses died, making four horses dead in the first 8 days of the racing season that lasts six weeks every summer; six horses have died since April. Statistics for Saratoga:

YearDead horses

According to the Gaming Commission of New York Executive Director Robert Williams, most of these deaths are attributed to excessive exercise-related, musculoskeletal injuries. Horses are also forced to exhaustion during training and races.

To sponsor these people by attending a race track is – for me – totally unacceptable. By spending money at race tracks we not only engross the pockets of the torturers of these magnificent animals, but also we encourage others to follow suit.

This is my opinion. I think if we like professional horse races, we may also like dog fights (that are getting more popular every day), or like to observe how a coward “torero” kills a bull with a spade in front of thousands of spectators, or even we may enjoy taking our kids to a zoo full of caged animals.

I wonder if we really need these types of entertainment. What is wrong with attending a Shakespeare festival in the summer?

What do you think about this?

125 comments; last comment on 08/14/2017
View/add comments

Bertrand Russell's Set Theory Paradox

Posted January 25, 2017 5:31 PM by amichelen
Pathfinder Tags: education Logic math paradox

Bertrand Russell

Gottlob Frege

In June 1902, Bertrand Russell, the great British mathematician and logician, sent the statement of a paradox to his friend Gottlob Frege, a German philosopher, logician and mathematician. Frege had been working for more than 10 years writing his monumental work “The Foundations of Arithmetic” and was finishing the final chapter of the second volume of this two-volume treatise. Russell and Frege had been friends for many years and Russell encouraged his friend to write a book about the mere foundations of arithmetic based on the set theory of Cantor. Frege obliged, but one day Russell found a contradiction of the work Frege was proposing in his book. This contradiction simply destroyed the very principle of his friend’s logic. Frege was devastated; his work of over ten years was simply irrelevant. Russell’s letter to Frege terminated the labor of more than ten years. Frege sank into a deep depression, while Russel tried to repair the damage by constructing a new theory of logic that would be immune to the paradox. He couldn’t. The paradox appeared again in his new theory.

The Russell paradox has been popularized in many ways. One of the best known of these was given by Russell in 1919 and concerns the plight of a barber of a certain village who has enunciated the principle that he shaves all those persons of the village who do not shave themselves. The paradoxical nature of the situation is realized when we try to answer the question: “Does the barber shave himself?” If he does shave himself, then he shouldn’t according to his principle; if he doesn’t shave himself, then he should according to his principle.

Since the discovery of the above contradictions, additional paradoxes have been produced in abundance. These modern paradoxes of set theory are related to several ancient paradoxes of logic, such as:

  1. Eubulides, of the fourth century B.C. is credited with making the remark, “The statement I am making is false.” If Eubulides statement is true, then, by what it says, the statement must be false. On the other hand, if Eubulides’ statement is false, it follows that the statement must be true. Then Eubulides’ statement can neither be true nor false without entailing a contradiction.
  2. Epimenides, who himself was a Cretan philosopher of the sixth century B.C., is claimed to have made the remark, “Cretans are always liars.” A simple analysis of this remark easily reveals that it, too, is self-contradictory.

The existence of paradoxes in set theory, like those described above, clearly indicates that something is wrong. Since their discovery, a great deal of literature on the subject has appeared, and numerous attempts at a solution have been offered. For some mathematicians there seems to be an easy way out. One has merely to reconstruct set theory on an axiomatic basis sufficiently restrictive to exclude these known antinomies. This is simply a procedure that avoids the paradoxes (putting your head in the sand.)

24 comments; last comment on 02/02/2017
View/add comments

Analog Filters Video Lecture

Posted December 07, 2016 12:00 AM by amichelen


In today's entry I am a introducing a presentation video related to the general topic of Analog Filters. The presentation is a general overview of analog filters, without the rigor of mathematical equations or design methods. This first video includes the following concepts:

· Definitions

· Signal representation

o Time domain

o Frequency domain

· Ideal filters

· Practical filters

· Filter types

· Frequency domain characteristics

o Butterworth

o Chebyshev

o Elliptic

o Bessel

· Filter realizations

o Passive filters

o Active Filters

· Specifications

Filters are electrical circuits designed to remove, attenuate or alter the characteristics of electrical signals; in particular these devices reduce the magnitude and the phase of unwanted signals with certain frequencies. For example noise (normally at a frequency of 60 Hz in electronics circuits) is always present, and it is desirable to suppress the noise from the system. We can achieve this by passing the system signal (voltage and noise) through a filter. If the filter is designed to suppress or attenuate the magnitude of the noise, the output of the filter will contain only (or mostly) the system signal. For another example consider a typical radio receiver (the radio in your car); by tuning to a particular radio station you are selecting one signal while attenuating the signals of the other radio stations. This process is accomplished by mean of a filter.

Several categories of filters exist, but the main distinction is between analog and digital filters. Analog filters are designed to attenuate signals in analog systems, while digital filters attenuate digital signals in digital systems. These notes will concentrate in the study of analog filters, leaving digital filter for another occasion. The study of filters entails the use of complex mathematical techniques such as z-transform, Laplace Transform, convolution, recursion, and others that will be discussed in later articles. This module present filter behavior without engaging the reader through advanced mathematics and complex techniques.

Types of Analog Filters

Filters are broadly classified according to the type of frequencies that the filter is able to suppressed or attenuate. In this regard, there are four main categories:

· Low-pass filter. This type of filter attenuates or suppresses signals with frequencies above a particular frequency called the cutoff or critical frequency ( ). For example a low-pass filter (LPF) with a cutoff frequency of 40 Hz can eliminate noise with a frequency of 60 Hz.

· High-pass filter. This is a filter that suppresses or attenuates signals with frequencies lower than a particular frequency - also called the cutoff or critical frequency. For instance a high-pass filter (HPF) with a cutoff frequency of 100 Hz can be used to suppress the unwanted DC voltage in amplifier systems, if so desired.

· Band-pass filter. A filter that attenuates or suppresses signals with frequencies outside a band of frequencies. This is the general type of filters used when tuning radio or TV signals.

· Band-reject, or Notch filter. A filter that attenuates or suppresses signals with a range of frequencies. For instance, we can use such a filter to reject signals with frequencies between 50 Hz and 150 Hz.

Response Characteristics

The frequency response of any filter (LPF,HPF,PBF,BRF) can be designed by properly selecting the circuit components. The characteristics of filters are defined by the shape of the frequency response curve; the most important response shapes are named after a researcher who studied the particular filters. There are filter of type Butterworth, Chebyshev (types I and type II) , Elliptic (or Cauer), and Bessel, to mention the most important. These filter types are named after the British researcher Stephen Butterworth, the Russian mathematician Pafnuty Chebyshev, the German scientist Wilhelm Cauer, and the German mathematician Friedrich Bessel, respectively. Each one of these filters types has a particular advantage in certain applications. The following figure shows the characteristics of four low-pass filters, each one of three-poles and cutoff frequency of 10. Note the different types of shape represented by the frequency responses.

The video presentation is located here: Analog Filters

Add a comment

Previous in Blog: Hypatia of Alexandria – Part 3  
Show all Blog Entries in this Blog