Lin Chaoxi remembers that when she was not good at math in high school, she was very afraid of encountering function problems.

At that time, Lao Lin was still unconscious in a car accident. She dozed off in class every day. She felt that the so-called definition domain a and the corresponding rule f were too abstract and difficult to understand. Sometimes she looked at the questions and didn't know what they were asking.

She feels a lot like that now.

What she had been experiencing was like a huge function problem. Lao Lin’s car accident might be what she required, “f(2)=”, but she didn’t know the analytical expression of f(x).

And now, Pei Zhi also seems to be the key variable in this function problem.

She comes from reality and is very clear about the properties of functions and the geometric images they draw. More precisely, she knows the ending of what everyone is experiencing now at a certain point in time.

What she had always wanted to do was to change that pattern.

But almost at the moment Pei Zhi walked through the iron gate, she suddenly realized that if she couldn't solve "this problem", she couldn't change anything.

This idea is very philosophical and involves the real existence significance of the two worlds. Lin Chaoxi realized this, put away her running thoughts, and looked forward.

Yongchuan Railway Station Ticket Hall.

Lao Lin took the ticket handed to him by the conductor.

The ticket has -

k796

"17:42" "Yongchuan→Anning", she looked up at the huge clock hanging on the wall. In about half an hour, they would embark on the return journey.

If he can get home safely, does it mean that the specific time and place where Lao Lin’s car accident happened have been changed?

The atmosphere at the train station was bustling; Lin Chaoxi bought two boxes of braised beef noodles.

She and Lao Lin stood in the standing dining area, tore open the seasoning, poured in hot water, and forked the noodles, waiting nervously for them to open.

The clock ticked by, and she looked at it from time to time.

Passengers poured into the waiting hall and formed a long queue. The ticket indicator of D198 turned green and she opened the lid of her instant noodles.

After eating two-thirds of the noodles, the passengers were almost gone, and D198 stopped checking tickets.

On the ticket gate display, k796 jumped to the next position.

Lin Chaoxi began to slurp the noodle soup. Old Lin came back from the toilet, ate a few bites, and put down his fork.

"Are you full?" Lin Chaoxi turned around and was shocked to see that there was still half a cup of instant noodles left in his bowl.

"I'm afraid you won't have enough, so Dad wants to save a bite for you." Lao Lin said lovingly.

"Did you buy roast chicken secretly?" Lin Chaoxi looked around sharply and sure enough, Old Lin was holding a bag wrapped in oil paper.

Lin Chaoxi quickly reached out to grab it, but Old Lin picked up the bag.

Just then, the phone rang.

At first there was a buzzing sound, followed by the sound of the piano bells that Lao Lin usually played. Lin Chaoxi tiptoed to grab the oil-paper bag, and suddenly stopped.

The air was instantly sucked out, and the surroundings were so quiet that there was not even a sound.

Lao Lin took out his cell phone. Lin Chaoxi watched him answer the call. He paused and then said:

"Hello, Professor Zeng."

It was like a stone thrown into the river, causing ripples and a sudden surge of noise.

"Passengers on train K796, please note that ticket inspection will begin soon."

The radio sound surged overhead, almost drowning out the sound of Lao Lin talking on the phone.

Lin Chaoxi could barely make out the general content of the phone call from Lao Lin's lips.

"Yes, I'm in Yongchuan and I'm at the train station now."

"Almost done, still revising."

Finally, he paused for three seconds to think, then he looked towards her and said, "Okay, I'll go over now."

Lin Chaoxi grabbed the oil-paper bag in a ridiculous posture and looked at Lao Lin nervously.

"What the hell? Eat yours. I won't steal yours." After Lao Lin finished answering the phone, he seemed a little relaxed.

"Who is Professor Zeng, Dad?" she asked tentatively.

"The grandfather who gave you a speech at the elementary school Olympiad summer camp." Lao Lin walked out of the waiting room with a look that said, "You already know the answer but still ask."

Lin Chaoxi's heart sank again. She took two quick steps to catch up. She could only hear the rumbling of voices in her ears: "Where are you going? The tickets will be checked soon."

"Sanmi University, they want me to go there and we can take a later train back."

She grabbed Lao Lin and blurted out, "Don't go. Let's go home quickly, shall we?"

"What's wrong? Are you suddenly homesick?" Lao Lin still looked relaxed, but he stopped and listened to her reasons very carefully.

How to say it?

Say "I'm worried that something bad might happen to you if you stay in Yongchuan", or "Let's go home and stay here and don't go out this week"?

But Professor Zeng's sudden call was obviously related to Lao Lin's thesis. How could she possibly ask Lao Lin not to go?

The ticket check indicator turned green and the gate began to let people in.

Lin Chaoxi looked back at the crowds of people in the waiting room preparing to leave Yongchuan, and realized that the huge function problem she encountered had a limited range of values ​​for solving it.

She looked at Lao Lin and felt a powerful force pushing them into the established track, but in the end she could only say: "Let's go."

——

Sanwei University, old campus.

There are large tracts of dense evergreen trees on the campus, which are eerie and gloomy on winter nights.

Lin Chaoxi and Lao Lin were standing outside the mathematics department building when they met Professor Zeng.

In the night, the old man with gray hair looked very energetic.

"Your daughter is already this big now?" Professor Zeng took out a steamed bun from his coat pocket and asked, "Are you hungry? Would you like to have a steamed bun?"

Along the way, Lin Zhaoxi's mind was tense and she felt cold in her bones, so she took the steamed bun and said rudely, "Thank you."

After a while, Professor Zeng took his eyes off the steamed bun and said to Lao Lin, "She is really your daughter."

"Okay." Lao Lin was very decisive. He took the steamed bun from her hand and broke it in two.

"Put away your midnight snack." He handed half of the steamed bun back to Professor Zeng and took a bite of the other half.

They walked upstairs and seemed to know each other well, but Lao Lin was so casual with most people.

The conversation between Lao Lin and Professor Zeng was neither salty nor bland, and Lin Chaoxi followed behind them.

As she approached the office door, she paused when she saw the people waiting inside. There were professors from the Mathematics Department of Sanwei University, two of whom she had seen in real life when she was a university student.

Only then did Lin Chaoxi realize that Lao Lin was invited to participate in the peer review before the paper was published.

For some unknown reason, Lao Lin sent the thesis to Professor Zeng for review, and Professor Zeng invited Lao Lin to personally explain it.

On that winter night, her heart beat violently, nervous and excited.

In this case, she certainly couldn't go in.

"I'll wait for you outside." she said to Lao Lin.

"Find a warm place."

"kindness."

——

Office of the Dean of the Department of Mathematics.

The yellowed blinds had been drawn. Lin Chaoxi turned around and returned to the office door.

Through the gaps in the broken blinds, she could see that there were only two chandeliers on in the office, and a stack of printed papers was placed on the table, but no one was flipping through them.

The room in the office was filled with smoke and there was a whole blackboard wall. Lao Lin was standing in front of the blackboard.

On a winter night, the cold wind blows through the leaves, making a slight rustling sound.

Lin Chaoxi held the strap of his schoolbag tightly.

Sitting in his seat, Professor Zeng raised his head.

He looked at Lao Lin with a serious and earnest expression, as if to say - "Let's get started."

——

Lin Chaoxi put his schoolbag on the ground and leaned against the office wall.

It was very cold at night and the corridor was like an ice cellar.

She was trembling as she sat down, rummaging through her schoolbag, thinking about what would happen next, and had not yet escaped the panic of being forced onto a predetermined trajectory by fate.

She found several sheets of manuscript paper with Pei Zhi and Lao Lin's handwriting in her schoolbag, a graph theory book she always carried with her, and her pencil case.

The winter night was silent and still. She looked up and saw a bright moon hanging above the city, clear and bright.

She sat down with her backpack propped up against her butt and opened the book.

——

The only sounds in the office were the sounds of people flipping through papers and smoking, very softly and slowly.

The smoke is bigger.

Lin Chaoxi spent much more time than usual to finish an exercise at the end of the book. Her hands were frozen, but her mood seemed to have calmed down a little.

She looked into the office again.

Most of the blackboard was covered with chalk words, but the atmosphere was stagnant. It seemed that someone was asking a question, and the question was very sharp and incisive. Everyone was looking at the questioner.

Lin Chaoxi looked at Lao Lin.

Her father was no longer young, and there was white chalk dust on his fingers, and even his cuffs were gray.

His eyes were calm, yet full of firmness and sharpness.

After hearing the question, he did not speak, but instead wrote on the blackboard.

One line, two, three, three lines...

Everyone frowned for a while, then showed some relief, as if they realized that this was a perfect solution.

Professor Zeng took a puff of cigarette and narrowed his eyes, as if to say - "Go on."

——

Lin Chaoxi sat down again and turned to the next page of the book.

The hour hand went one circle, and then another.

The ashtray on the table was filled with cigarette butts, and another new one was put out.

The blackboard has been wiped several times and all the unevenness is about to be filled.

Lin Chaoxi was still looking at the question on his knees——

(2) Write the neighbor matrix of g.

She thought about it and continued writing.

——

It has been said that most of the most important achievements of mathematicians are made before they are 40 years old.

Mathematicians are no longer useful after the age of 40.

Now, Lao Lin is stuck at this juncture.

In the office, one problem after another was raised and then solved.

Many mathematicians walk alone in darkness throughout their lives. How many people are lucky enough to experience the moment when light pierces the darkness?

The landscape lights on campus were completely turned off, and the cedar trees swayed in the cold wind.

Lin Chaoxi put down her pen, clenched her palms together, and let out a breath.

At this moment, a slight "click" sound was heard in the space.

Lin Chaoxi turned his head suddenly, and the light seeped into the corridor through the crack of the door like a stream of water.

The door is open.