Subgraph isomorphism is an NP-complete problem, while the graph isomorphism problem has not yet been proven to be a P or NP problem.

She had probably heard some content about this in a lecture that Pei Zhi had translated.

A staircase divides the Mathematics Department building of Yongchuan University into two halves.

On the other half of the aisle, workers were carrying cardboard boxes and cleaning offices.

Lin Chaoxi read it for a while. In fact, she really wanted to find a computer to search for detailed papers on the subject, but now she still had to concentrate on the answer in front of her.

"Let's go over the defense process again?" Lin Chaoxi turned around and said to Lao Wang and A Guang.

The two were confused: "How did you change so quickly?"

"Because I'm a woman." Lin Chaoxi replied absentmindedly.

Before entering the defense classroom, Lin Chaoxi could still joke with Lao Wang and A Guang, but when she pushed open the door of the defense classroom and was shocked by the cold air in the classroom, she understood why Li Ran was crying.

There were two rows of teachers sitting in the audience, not many people actually. But they all looked serious, and the lights in the old campus of Yongchuan University were dim, and the windows were mostly covered by ivy, which made people feel depressed.

Lin Chaoxi had anticipated the difficulty of the defense, so she didn't feel too nervous. After bowing and greeting, she inserted the USB drive and opened the PPT.

"Good morning, teachers and students. Our group number is 205912. The topic of the modeling competition is B: Suspect Tracking.

She had rehearsed the main content of her speech several times before, so she started analyzing the problem and explaining the main points smoothly.

After 7 minutes, she finished her statement.

There was silence in the audience.

"I know it very well." After a while, a female teacher looked up and said lightly.

When Lin Chaoxi participated in the defense of his graduation thesis in the Department of Philosophy, he also received such a comment. But the expressions and attitudes of their department teachers and the current female teacher were completely different when they spoke.

Just from the look in the female teacher's eyes, Lin Chaoxi could tell that she probably thought she was there to win the prize for free.

"Because I have been preparing for a long time." She answered seriously.

The female teacher raised her eyelids again, frowned, and said unhappily: "Then tell me more about your modeling process for the second question."

Lin Chaoxi took a half step back and let Lao Wang step forward.

"Well, according to Bayes..."

As soon as Lao Wang's voice sounded, he was interrupted: "What did you say? I'll let her answer."

The female teacher looked stern.

Li Ran had probably just encountered a similar situation, so he was so nervous that he couldn't say a word.

"Wang Tianlong was responsible for the derivation of the overall mathematical modeling. Lin Chaoxi paused, "We think it would be more appropriate for him to answer."

"If you are a member of the team, why don't you understand the modeling formula? Even if you are just an ordinary mathematics student, you should be able to use the Bayesian formula easily." The female teacher paused, "Are all the mathematics students in Sanwei University at your academic level now?"

"They are all better than me. I'm from the philosophy department." Lin Chaoxi replied.

"Although the purpose of the competition does not explicitly state that cheating is not allowed, but now anyone can participate in the modeling competition?"

"What did you do?" Lao Wang looked at her and suddenly asked.

"Teacher, the entire thesis structure was proposed by Lin Chaoxi. This is the only math ability we have in the philosophy department of our school. It's not great, but it's OK. She is my apprentice and is preparing to take the math graduate entrance exam. She's not just taking the prize for free."

It was rare for Lao Wang to speak seriously, but it always sounded like a mockery.

"Since she wrote the thesis framework, why can't she answer even the simplest model deduction?" The female teacher suddenly looked very angry. "A philosophy major took the math postgraduate entrance exam? You probably don't know that math is different from the humanities you studied before. It requires a lot of solid calculations and hard practice. All the foundations are in the question I just asked. It's not good to have only a superficial understanding!"

"I understand." Lin Zhaoxi picked up the chalk, "Let me answer."

She faced the blackboard and began to write.

Although there is definitely a sentence in the guide that says not to conflict with the teacher, it has come to this now and there is no point in saying anything.

Lao Wang was in charge of the derivation of the overall model, but these were all very familiar contents to her.

Starting from the basic Bayesian formula, she wrote down the entire modeling process and even completed many omitted parts in the paper in detail.

In order to avoid further questioning, she also briefly deduced the algorithm they used.

These contents are actually very basic and not very profound, but all proofs and deductions test the degree of understanding of the problem.

Finally, she faced the entire blackboard filled with chalk words, put down her pen, and turned around.

Lao Wang and A Guang had sparkling eyes and secretly clapped their hands in the area hidden by the podium.

The teachers in the audience all half tilted their heads back. Although they did not say anything, their eyes no longer showed the indifference and carelessness they had before.

There seemed to be a standoff between the audience on and off the stage.

Finally, the female teacher who had called on her to answer the question pushed up her glasses, as if she had reviewed all the answers, and she lowered her head to write down a score.

Lin Chaoxi bowed, packed up his things and prepared to leave.

"Wait, Lin Chaoxi, right?"

Lin Chaoxi stopped all movements and looked in the direction of the sound.

"The last question is, why did you switch from philosophy to mathematics? You suddenly decided to cross the exam in your senior year. Don't you think your life is too casual?"

He was a middle-aged male teacher sitting in the last row of the classroom, with sharp eyes that seemed to be able to see through everything.

Lin Chaoxi pursed her lips and wanted to answer something, but suddenly she didn't know how to answer.

At that moment, she didn't know what she was thinking, but she suddenly wanted to know the other person's answer.

She took out WeChat, lowered her head, and sent this question to Lao Lin at the podium.

"You may think that Mr. Zhang is too strict with you, but in our opinion, you take mathematics too lightly. You think it is a proud thing that you can easily jump from philosophy to mathematics with your intelligence, right?"

"No." Lin Zhaoxi denied.

"Then answer me directly, do you like math?"

"It's not like or dislike." Lin Zhaoxi put his phone back into his pocket and looked up and said, "I didn't choose mathematics, mathematics chose me."

——

Walking out of the classroom and down the stairs, Lin Chaoxi passed by workers carrying cardboard boxes.

Why did she switch from philosophy to mathematics? This was a question many people asked her after she came back to the world for the second time.

For her, there were too many twists and turns involved that she couldn't tell anyone else.

But deep down inside her was there no answer?

Actually, there is.

There is no reason, it’s just because Lao Lin has been training her in this direction since she was young.

When she came to the crossroads of life in her senior year, she suddenly felt anxious about the future. However, she happened to get a chance to start over again, and e=mc^2 pushed her onto another path in life.

She seemed to just do that.

"Don't be too sad." Lao Wang patted her shoulder and called her back from her long memories. "It's quite common for science and engineering guys to look down on girls."

Lin Zhaoxi suddenly felt like laughing: "Master, aren't you a science and engineering guy?"

"I'm different. I look down on both men and women."

"Master, why do you want to learn mathematics?" Lin Chaoxi asked Lao Wang.

"How can ordinary subjects satisfy the genius of teachers?"

"Is mathematics the only thing that can satisfy you?

"Of course, physics is also acceptable."

"You are too modest..."

The summer breeze blew on his face, the vines swayed, and Lin Chaoxi's mood suddenly improved.

Essentially, the male teacher said that to discourage her. In fact, she doesn't need to care too much about what these people say.

After all, there is no shame in admitting that you are not the kind of person who knows what you want to do from birth.

At this moment, her cell phone vibrated.

Lin Chaoxi turned on the screen and saw the message she had just sent and the reply from Lao Lin.

——Dad, why did you study mathematics?

——There is no reason, mathematics chose me.

Lin Chaoxi stood still, holding his cell phone.

After saying that, Lao Lin also sent an expression that said “I am the best in the world”.

At that moment, she could almost imagine Lao Lin's wanton and arrogant face.

His answer was almost the same as hers, yet completely different.

Because Lao Lin’s answer is the answer that truly belongs to the chosen genius.

Lin Chaoxi put his phone back into his pocket and looked back at the debate classroom, feeling only regret.

In the numerous universes, is there a parallel time and space without her?

At the same moment, Lao Lin in that world was one of the professors sitting at the podium, picking on students. A second ago, he had just finished drinking a mouthful of water, and now he was ready to continue spraying.

Likewise, the Lao Lin in that world also fulfilled all his wishes and aspirations and had extremely high academic achievements, which was completely different from the Lao Lin in this world who had to work hard to earn money to support her.

It was around that time that Lin Chaoxi stepped down the last step.

Hundreds of cardboard boxes were lined up at the bottom of the stairs. A truck was parked at the side of the building and workers were moving things.

The cardboard boxes were filled with countless mathematics department calculation papers and archival materials, most of which were dark yellow and old.

At first glance, it seems to be just an insignificant tidying up and moving, but when you start to guess how many flashes of human thinking are recorded here, you will still be shocked.

Lin Chao scanned a cardboard box and found a manuscript with mathematical proofs and deductions that she could not quite understand. At that moment, she saw a string of numbers.

She couldn't tell whether it was a coincidence or the hand of fate finally slid to this moment.

The manuscript paper reads: Institute of Applied Mathematics, National Yongchuan University.

Below is the institute’s address and contact information – 021-57632376.

She took two more steps before stopping. She turned back incredulously, took a few quick steps to the cardboard box, and squatted down to check again.

021-57632376

Because of the long time, the manuscript paper seems as fragile as the pale yellow wings of a moth, but the serial number is as bright red as blood.

In the cheese city of Anning, under a huge camphor tree, the fifth grade girl saw Lao Lin dial a number.

Because of the glare of the sun, she did not see the last two people, and therefore regarded it as fate, and did not explore as much as Lao Lin did.

At this moment, the sunlight was as dazzling as that day. There was no camphor tree above her head, but a huge mystery enveloped her instead of the shade of the tree.

Yongchuan University, Sanwei University...Why did Lao Lin finally give up mathematics?

At that moment, Lin Chaoxi didn't even need any time to calm down. She looked up and asked the worker guarding the cardboard box: "Are these from the Mathematics Department of Yongchuan University? What are you doing? Where are you shipping the things to?"

"We were only responsible for moving things to the new campus. We knew nothing."

Having said this, Lin Chaoxi was suddenly pulled by someone. Turning around, she saw Lu Zhihao's puzzled face.

"What's wrong with you? What happened?"

Lin Zhaoxi held Lu Zhihao's arm and asked, "Does the medical program take five years?"

"No, I'm going to study for a master's and a doctorate degree simultaneously, which will take eight years."

"Then your campus card can still be used, right?" she said, pulling Lu Zhihao and running away.

"Where are you going? What's the matter?"

"Go to the archives room of your school library."