The return journey was long, with lush green French plane trees on both sides.

Lin Chaoxi was walking in the shade of the trees, carrying a backpack. It was a long distance from the hospital to the university town lecture hall, and she could take a taxi or a bus. But after spending an entire afternoon in the hospital, the lecture time for the basic university mathematics course had long passed, so she simply didn't rush and walked back slowly.

The summer breeze is gentle, young boys and girls are walking by, their black hair fluttering in the wind, making you want to eat an ice cream.

Lin Chaoxi licked the corner of his mouth, turned left and walked into a convenience store.

She stood in front of the refrigerator full of goods for a while, wondering whether to eat a strawberry-flavored Baskin-Robbins or a crispy Monero.

The sliding door opened with a ding, and Pei Zhi walked in.

Lin Chaoxi looked at him, and his first reaction was what kind of weird fate was this.

Pei Zhi was on the phone and saw her. He nodded politely and walked straight to the water cabinet. He had a clear goal in mind. He took a bottle of mineral water and walked to the cashier. After the beeping sound of the cashier, he took out his money to pay and left.

That was probably the scene that kept replaying in Lin Chaoxi's mind later.

From beginning to end, Pei Zhi was talking on the phone. He spoke in low and fluent English, content that she could not understand at all, and because of his humble and casual attitude, there was no boasting at all.

He left the cashier, tilted his head slightly, held his phone between his shoulders, reached out to twist the bottle cap, and took a long sip.

She was clearly right next to him, but he had no intention of asking someone else to do it for him. Before leaving, Pei Zhi even nodded to her again.

The sliding door opened and closed, and he left the convenience store and walked into the shade of dense green trees.

Lin Chaoxi looked at the boy's handsome and tall back, watching him walk away until he disappeared.

The cool air of the convenience store's air conditioning fell from the vent. Lin Chaoxi retracted her gaze and looked at the dazzling array of cold drinks in front of her. Whether it was VC drinks or WeChat, all the girlish heart she had just had disappeared.

She didn't think there was anything wrong with Pei Zhi not stopping to hang up the phone and talk to her. They were just nodding acquaintances, which was the normal distance between them now. She had had the opportunity to pass by Pei Zhi countless times before, and she always passed by like this. In fact, it was nothing.

Lin Chaoxi opened the freezer and exchanged for a strawberry cone with the sweetest taste to comfort herself.

——

It was not until he sat down in Report Room 1 and the lights dimmed that Lin Chaoxi roughly understood who Pei Zhi was calling.

This was the third time she saw Pei Zhi today.

The young man changed into a formal smoke-gray suit and a white shirt, with his sleeves rolled up and no tie, looking very handsome and clear.

He seemed to have the school badge of Sanmi University pinned on the front of his suit, and was leaning forward slightly to adjust the projector with the staff in the lecture hall. He was focused, occasionally talking to the staff beside him, and the slides on the screen moved and stopped, and soon returned to the original page.

——p/np and the search for the impossible

by andy howard

There are two lines of white text on the blue background.

The speaker, Mr. Howard, is a professor at H University, a mathematician, and a senior writer. He has in-depth research on the p/np problem. This is already noted in the lecture promotion, but what is not noted in the lecture promotion is that the translator of this lecture is Pei Zhi.

The seats below the stage were dimly lit. Lin Chaoxi's hands were resting on the partition of the seat. Next to her was a mobile phone that had been switched to silent mode. On the screen, the college WeChat group kept refreshing the on-set photos taken by the fans on the playground.

Hanamaki was wearing a soccer jersey and, under the bright lights on the set, he was posing for shots continuously, and he looked dazzling.

Lin Chaoxi took one last look at his phone, turned it over and looked up.

Six o'clock.

The academic lecture did not have any complicated opening ceremony. Pei Zhi put on the microphone, went on stage, and stood right in front of the curtain.

He bowed slightly and greeted the audience. His face was illuminated brightly by the projector spotlight, making his straight nose and handsome features stand out more clearly.

The peaceful and gentle voice flowed out: "In 2000, the Clay Mathematics Institute in the United States announced seven major mathematical conjectures and offered a reward of 1 million US dollars for each difficult problem in the hope of finding the answer. These are the peaks that countless people in the mathematics community are eager to climb. The solution to each problem is destined to change the course of mathematical development. The p/np problem is also among them. Today we are fortunate to have invited Mr. Howard, a tenured professor of the Department of Mathematics at H University, to introduce this world-class mathematical problem to everyone."

Pei Zhi's introduction could not have been simpler, but the audience burst into applause and he naturally stepped aside.

There are no fangirls here.

The lecture was of a popular science nature and welcomed people from all walks of life. Lin Chaoxi sat almost at the end of the hall. Looking around, he saw many elderly people with gray hair and young students. Among them were teachers, professors, and students wearing the uniforms of Yongchuan High School. There were papers and pens on the partitions of each seat, and everyone was looking up and applauding.

Professor Howard, with a big belly, came on stage amid applause. He stood in front of the slides. As the applause gradually subsided, he greeted everyone present with a smile and introduced his translator Pei Zhi in the same graceful manner.

The lecture hall fell silent again, with only the light from the podium.

Professor Howard began to talk about the definition of the p/np problem. In the popular science lecture, they did not talk about complicated mathematical definitions, but explained it with very simple concepts.

If p=np, it means that humans can quickly calculate the solution to any problem.

He started with an ideal cancer treatment, describing a treatment method that does not require chemotherapy but can identify the DNA of normal cells and cancer cells through DNA testing, thereby producing specific proteins that effectively starve the cancer cells to death and eliminate them from the body.

This new treatment option will make cancer treatment more efficient and targeted, will not cause side effects to cancer patients, and more importantly, it will be cheap, making it affordable for every cancer patient.

The idea of ​​this treatment plan first relies on an algorithm that can effectively solve the np problem, namely p=np.

Lin Chaoxi had no problem understanding simple everyday English. As Pei Zhi translated it, Lin Chaoxi gradually began to see that beautiful world that was simple and efficient.

She finally understood what Lao Lin said. If such an algorithm appeared, then Alzheimer's disease, which is caused by the deposition of β-protein, should also be cured.

The slide showed a scene of cheers and excitement, and all the audience began to applaud and laugh.

Only on the lecture stage did Pei Zhi and Professor Howard look at each other, with clear and calm eyes.

The following explanations were more in-depth, from P problems, NP problems to NPCs, from time complexity to the polynomial level of time complexity. The lectures began to extend into the real field of mathematics. Lin Chaoxi could understand some of it through Pei Zhi's translation at first, but gradually, she felt exhausted.

Although Professor Howard had found many examples to explain a lot of content, Lin Chaoxi knew very well that she simply did not have the knowledge reserves to understand those professional terms. She had been away from the world of mathematics for too long and was even worse than an ordinary high school student in some aspects.

As the session progressed, the expressions of both the speaker and the translator became calmer and clearer.

The final extended and in-depth content was too strenuous for her, so some bright images overlapped and she began to lose focus.

I always feel that there seems to be a similar scene in my memory. At that time, she and Pei Zhi were working side by side. Pei Zhi was still young, with a fair and delicate face. He looked at the old man on the projection screen with a confused but yearning look.

Time goes backwards.

Ten years later, she was still sitting in the audience, while he was already standing on the stage.