A set is a basic concept in mathematics, a set is "a definite bunch of things", and the "things" in a set are called elements. A modern set is generally defined as a whole made up of one or more definite elements. Wen Yifan held the chalk in his right hand and wrote, "This is what we are going to learn next, do you have any questions?" ”

"Teacher, is the thirteenth question of the last exam paper serious?" A complaining voice came, "I asked the students in other classes, and they all said that there is no such question, and it can't be added by the teacher himself!" ”

"Liu Mingyu, 1.81 meters tall, weighing 132 pounds, middle and lower grades in academic performance, loves sports and street dance." The information about him quickly swiped through his mind, and Wen Yifan smiled back.

"That's a good question, this topic was indeed added by me privately," Wen Yifan tapped his finger on the podium, "Does anyone know why I added this topic?" ”

"To open up your mind?" Ning Yufang asked tentatively.

"No, for the sake of better interpersonal communication in the future." Wen Yifan smiled and shook his head, looking at everyone's puzzled eyes, he explained: "Life after you...... No, you are communicating with different people, and it can be easy or difficult, and I have been studying this for some time. ”

"I don't want to be like other people to hypocritically tell you that you should be sincere in the process of communication, in fact, in my model, this kind of communication is the most difficult to determine in the world, it may have information asymmetry, inconsistent goals, inconsistent thoughts or even just simple emotionality, these factors will make it difficult to interpret the meaning of the other party, and there are countless deceptions, and some of the topics I have added are to help you better deal with these exchanges." Wen Yifan explained to the whole class, while writing on the blackboard "A1z+B1=A2y+B2=A3x+B3=A4k+B4", and then drew six vertical lines and split them into three functional relations, "Sometimes, in the face of a complex communication, we may only need to disassemble it one by one to observe the connection between each of these factors, and we can easily simplify the complex, so that it is extremely easy to classify and establish a connection between the originally incomprehensible complex exchanges. It becomes something that you can understand at first sight. ”

After a minute of silence, looking at everyone's thoughtful looks, Wen Yifan smiled: "And the collection to be learned today is another extension of this kind of exchange, or in other words, it can give this kind of exchange a certain premise." ”

"For example, when you are walking on your way home and you are confronted by two people, one is your friend and the other is one of your relatives of the same generation or older, which one would you prefer to communicate with to send the way home?" Wen Yifan pushed his eyes, "I think, everyone should choose the former!" ”

"Hahahaha-" The students laughed, and the atmosphere in the class came alive again.

"So, even though we may have a closer relationship with our relatives than our friends, when we choose an individual to communicate with, we still choose the friend who has a lot in common with us, and then this 'person with a common topic' becomes a collection, and you and your friends all belong to the elements of this collection, and relatives are excluded." Wen Yifan drew a circle, putting the words "I" and "friend" together, and separating the word "relative" separately.

"Teacher, shouldn't the collection be deterministic?" People with common topics are so broad that the certainty of such an explanation is lost soon? Questioning voices came.

"It's true, it's not rigorous to say this, but you must know that this model itself is based on the vague foundation of communication between people," Wen Yifan's gaze swept across the classroom, "On this basis, the mathematics that was originally true and false has also become blurred, so we have a course in social psychology, not social mathematics." ”

"Therefore, when the set located in social psychology loses its own certainty, its other factors are no longer so certain, and even every element located in it may leave the set at any time, and the above-mentioned communication chain is one of the important factors that determine whether the elements can continue to exist in this set, and usually the detachment of elements from the set due to this reason is nothing more than a change in emotion, or in other words, a change in psychology." Wen Yifan smiled, "Of course, our topic seems to be a bit far away now, math class is about to become a psychology class, if you are interested in what I mean, you can go home and recall some of the things you have communicated with others in your life to carefully understand 'A1z+B1=A2y+B2=A3x+B3=A4k+." B4' This chain of relationships, because it has another meaning, is the most effective means of gaining the upper hand in communication with people, and these can only be understood by you. ”

"So, going back to sets in mathematics, turn to page 17 of the textbook, and let's explain the various properties of sets."

Wen Yifan opened the math book and pulled everyone's thinking back to the classroom again, and the hidden meaning turned into two blood-red words, which were impressively marked under the chain of relationships.

Control.

………………

September 25th

"Click-" The shutter sounded, and the reporters outside the cordon kept filming something, while the police on the periphery formed a human wall to stop the reporters who might have done anything to disrupt the scene.

The second murder of a police officer.

Killing, digging out the heart.

The forensic doctor is on his way, and the first task of the criminal police officers present is to protect the scene.

"Wow, it's another case, what a hate!"

"It's so cool, Yunhe Daxia is really a hero! Needless to say, this cop is definitely not a good thing! ”

"That is, Yunhe Daxia is not going to let go, he will kill one person in three days, until he kills enough 100 policemen! If there are still evil cops, then kill 100 more! ”

"I still hope to catch the murderer as soon as possible."

"There's a problem with my brain upstairs."

…………

Many such words are heard endlessly on the Internet, and some of the pressure from the police in the past life can be gathered at this moment, and it may not be obvious that these malice in the face of the police are taken out alone, but when the malice of thousands of people converges at one point, the torrent formed can crush all rational thinking and erase the goodness of a certain existence, and the irony is that these malicious pressures are almost all caused by the parties themselves, such as parking violations, such as petty theft.

Wen Yifan drew a circle and circled the word "malicious", while flipping through the messages on the Internet, most of which were insults for no reason, but there were still voices questioning and abusive and a very small number of reasoning, although the comments below were just quarrelling.

"From the current point of view, it is likely that the murder is not because of some ironic reason, but more like a personality perversion, that is, what we often call a perverted serial killer, the common characteristics of this kind of killer are animal abuse, arson, irritability and are very likely to return to the scene after committing the crime, hoping to contribute to the detection of the case."

"Back to the scene?" Wen Yifan couldn't help but laugh out loud when he saw this reasoning.

This is impossible, this is a mistake they will never make, whether it is Wen Yifan or the so-called "hero".

This is the mistake that every trained killer does not make - after making sure that he did not leave any contact with the victim in the process of killing, then from now on, even if the body is found, he must ensure that he does not establish a new connection, even if this connection is one-way, even if it is just a glance from a distance, this connection is fatal, and it may become the possibility of exposure one day.

Except for this reasoning, after Wen Yifan's gaze stayed on it for a while, he almost swept away all the rest of the comments, not wanting to waste a little time on it.

After repeatedly watching most of the news editions and the comments below, Wen Yifan closed his notebook, still picked up the lantern, and walked into the basement.

Looking at the clean and tidy basement, Wen Yifan smiled with satisfaction - the finishing work went smoothly, and there was no witness in the whole process, which meant that his workload would not increase due to some boring accidents.

Tearing off a few cylinders of plastic film, Wen Yifan slowly spread it out, covered the basement floor with a thick five-layer plastic film and carefully inspected every corner, and after making sure that there was nothing missing, he hung the plastic film that had been cleaned and made into a curtain on the roof and hung down, tightly surrounding the entire dissection table.

Finally, he took out a washed, transparent raincoat and a pair of black leather shoes, and placed them on the table next to him, on which he placed a comb, a pair of scissors, a camera, a clipper, and a sharp scalpel—the nearby barbershop was a place he frequented.

In the end, Wen Yifan, who had completed all this arrangement, took a deep breath, and the air in the basement was inhaled into his nose, and there was no peculiar smell in it - the effect of activated charcoal was still so good.

Turning over and lying on the dissecting table, he looked at the medical lamp facing his face, carefully examined every corner of it, and smiled when he finally found the slightest trace.

Slowly closing his eyes, carefully thinking about every detail, after confirming that he had no other omissions, Wen Yifan got up and walked down the dissection table, picked up the lantern and walked out of the basement.

New guests are coming in soon, and as a homeowner, you need to make sure that your room is clean and tidy.

Wen Yifan changed into casual clothes and applied in front of the makeup mirror, and a balding old man with dark skin soon appeared - silicone is always so easy to use.

Bending his back slightly and adjusting his temperament, Wen Yifan went out.

Let's start with the crime scene! He thought so.