In the middle of the curtain, a smiling old man appeared.

He was wearing a very plain navy blue work suit, with a white wall behind him. He was sitting on a sofa, and his smile was as plain as his clothes, making him look amiable.

He didn't have a script, but his impromptu speech was completely different from that of Vice Principal Zhang. He didn't seem to have planned what he was going to say. He just looked at the camera and said casually, "Dear students, I'm sorry to meet you in this way. It seems distant, but I think this distance should make both of us feel relaxed."

The old man was very sincere, and all the students in the auditorium looked up at his face with relaxed smiles on their faces.

He said: "I am actually a very shy person. It takes me a long time to prepare myself mentally before I can speak in front of many people. So the organizing committee suggested that I could use video to convey my feelings to you. My first reaction was, oh my god, this is really great."

His exaggerated smile of genuine joy infected all the children in the audience. Both young and old children burst into laughter. The old man himself was also laughing, with a kind look and sparkling eyes.

After a while, he continued, "Just now, you other teachers must have said a lot about the significance of this summer camp, the purpose of learning, and all the things that can and cannot be done. I am grateful to them for saving me from repeating these very redundant matters. So what should I say? Well, in fact, I won't hide it from you. Every time I make a similar speech, my first impulse is to tell you how much I love mathematics. I will tell you without any reservation that I love it very much. It is so delicate, wonderful, and profound... But it is such an interesting thing. No matter how much time I spend describing it to you in complicated language, it is far from enough."

The old man was in high spirits, and when he talked about mathematics, he smiled even more brilliantly, as if there was a fire in his chest, infecting everyone in the venue.

"Let me put it this way. When I was very young, my father told me a picture while he was coaxing me to sleep. Imagine that at the beginning of this world, it was pitch black. The so-called beginning is a very long time ago, long before the birth of humans, long before the birth of dinosaurs, and long before the formation of the earth. And the so-called black is not the black that can be painted with any kind of seasoning you have seen. There was no light. There was no light at that time. Then something happened, an extremely A tiny, dense point exploded, and it turned from the inside out. Time and space were spread out. The world we know and don’t know began at that moment. They kept extending. It was the extension of the entire time and space, and it took countless years. Long after that, small groups of gas gathered and gradually became brighter. We call it the Milky Way. Later, the sun appeared and the earth was formed. Then countless years later, on a very small planet, someone opened his eyes. ”

The old man's voice gradually became calmer. He squinted his eyes slightly, and his voice was as pleasant as piano keys: "That person is you, and the time is very deep in the night. You look up and see the whole wonderful starry sky. The stars are shining and magnificent. Someone told you that it is called the universe."

On the screen, the old man still smiled with a radiant smile. But there was no sound in the entire auditorium. It was quiet all around. All the children looked up. They were looking at him and the world he brought. The surroundings were as silent as the universe.

The silence of thinking lasted for a while, and the old man smiled: "When I was a child, the picture my father described to me really made me feel very beautiful. The real world I live in actually came from this, which is really interesting. But later, I went to high school and college, and I gradually gained more knowledge. At that time, I realized that every word my father told me when I was a child could be truth or fallacy. It is a kind of speculation about the origin of our universe, which is still being improved. Maybe one day it will be confirmed, or one day it will be overturned. This is all possible. You see, in fact, every day we stand on the edge of the known and the unknown. If you look at it this way, the world is too big, and we know too little, everything seems uncertain, which is very frustrating. But what I want to tell you is that the unknown is the most wonderful thing."

When the old man said this, he sat up straight, moved closer to the camera, and his face became more energetic.

“Human civilization has gone through a long dark night, and at a certain moment in the dark night, sparks of thought collided, and our ancestors created the number 1. Starting from 1, human abstract logical concepts were formed. If the world we originally saw was so big…” The old man smiled and put his thumb and index finger together, making a very small gesture.

He smiled and said, "After that, we gradually realized that the world is always bigger than we imagined. For thousands of years, countless predecessors have continued to improve this subject. It is no exaggeration to say that it is mathematics that opened our eyes and gave us the opportunity to see the true face of the entire unknown world; it is mathematics that built a bridge between seeing and knowing."

“Mathematics is a tool and also a language. You may find it hard to master this tool or language, but believe me, compared with the joy of discovering the unknown, all these efforts and hard work are worth it. Once you realize that we are just tiny humans on a tiny planet, but we are trying to master something that can understand the truth of the universe, you will suddenly realize that your learning process itself is already amazing enough!”

The old man paused, he licked his lips with a smile, as if he felt he had said too much in one go and felt a little shy.

Lin Zhaoxi looked around again.

Pei Zhi had long since woken up, and he was looking up at the old man on the screen. His eyes were still clear, but he had a new look in them, the same as the old man's. If from now on, you wanted to become his student and worked hard for it for many years, it would be a great thing.

Lin Zhaoxi smiled from the bottom of his heart.

The old man's last voice came from the speaker. He said, "If you encounter any problems in learning, you can ask your teacher or send me an email. I sincerely hope that you can feel the joy of learning mathematics and I sincerely wish you to have a good time in this process."

An email address appeared on the screen and the picture gradually went dark. Some people started to applaud, but most of the auditorium was still in a deep silence.

But suddenly, the old man in the picture looked up and laughed again. He faced the camera, patted his head and said with a smile: "I forgot to say that I prepared a small question for all of you. If anyone among you can solve it, you are welcome to send me the answer through this email. But believe me, even if you solve this question, you cannot immediately represent the competition, and you will not get any reward. This is just a delicious dessert after your hard study every day. I hope you can taste it a little. It is very interesting."

The old man's face finally disappeared, and a cross chessboard appeared on the screen.

The question was also typed out at the same time:

According to the rules, how many steps are needed to clear all the pieces except the middle position? List your steps.

In front of the auditorium, staff members had already stood up and carried a few inconspicuous boxes. They took out small wooden boxes from them and distributed them.

The children who received the wooden boxes were very excited. They didn’t expect to receive gifts at the meeting. Some of them shook the boxes vigorously, while others opened them directly and laughed in surprise.

The chess boxes were passed down from the seats on the left side one by one. Lin Chaoxi took a box and handed the remaining stack to Pei Zhi. She opened it and found that it was a cross chessboard exactly the same as the one on the curtain.

There are 33 depressions in the cross chessboard, and 32 of the depressions are filled with chess pieces, with only the middle square being empty.

There is a small piece of paper inside the chess box stating the rules.

1. After all the pieces are placed, take out the one in the center.

2. A chess piece can only jump horizontally or vertically (not diagonally) over its neighboring chess pieces to an empty space.

3. The chess piece that is jumped over will be captured.

4. Continue moving according to the previous method until you can no longer move, and count how many pieces are left.

5. The fewer chess pieces you have left, the more powerful you are. If there is only one left, then you are a genius.

Lin Chaoxi looked up at the question on the screen again - according to the rules, how many steps are needed to clear all the chess pieces except the middle position?

She looked at the young boy beside her again and remained silent for a long time.

Pei Zhi had already opened the wooden box and started fiddling with it. The light was dim, and his fair and tender face showed an unprecedented excitement.

Suddenly, Lin Zhaoxi recalled a moment that happened in the future, when the adult Pei Zhi stood on the steps and asked her two terms she had never heard of before.

"Fisher linear discriminant function?"

"Complete solution category?"

At that moment, she stroked the warm wooden chess box in her hand, as if she realized something.

Kongming chess, optimal solution, traveling salesman problem... Fisher linear discriminant function, classification of complete solutions, PNP problem...

Everything starts from here.