When leaving the classroom, Lin Chaoxi felt quite relieved.

What can I do if I don’t study math well and can’t solve the questions given by my idol?

But on the way home, the bus was too crowded and she felt a little depressed after getting off the bus.

It was completely dark.

A barbecue stall has been set up outside the new village, with smoke rising up and the aroma of lamb skewers and grilled scallops filling the air.

Standing in front of the familiar barbecue stall, her mind was full of Pei Zhi's back as he left. She was also very hungry, so she sent a WeChat message to Lao Lin asking if he wanted to come down and eat together.

Lao Lin responded quickly - [Great].

Holding her phone, she waited at the upwind entrance and ordered what they usually ordered. But the food was half baked and there was no sign of Lao Lin at the entrance of the new village. Looking at the dark and lonely iron gate, Lin Chaoxi suddenly had a bad feeling in her heart.

Her hands were shaking when she dialed Lao Lin's number.

There was a lot of noise all around her, but she could only hear the "beep...beep..." waiting tone on the phone.

Her heart sank to the bottom of the water. What was wrong with her brain that she wanted to go downstairs with her dad to have a barbecue!

After no one answered the phone once, Lin Chaoxi decisively gave up calling again. She greeted the owner of the barbecue stall and ran to the entrance of the new village.

When we were chatting on WeChat, Lao Lin was still at home. If Lao Lin didn’t leave through the back door, he was most likely still at home…

But why didn't you answer the phone?

In just a few minutes' journey, countless possibilities flashed through Lin Chaoxi's mind.

She ran to the door of her house, and suddenly saw a leisurely figure under a tree.

The old man was wearing a sweatshirt and plastic slippers, and he was playing with a passing stray cat with one hand out of boredom.

Moment...

Countless air rushed into his chest in an instant, and Lin Chaoxi's legs went weak and he almost fell to the ground.

After adjusting her mood, she walked up to Lao Lin.

Lao Lin looked up, very surprised: "Why did you come back? To lose weight?"

Lin Chaoxi had a bad feeling. It seemed that Old Lin had completely forgotten about their barbecue appointment.

"Why are you here?" she asked tentatively, choosing the right sentence.

"I forgot to take my keys when I took out the trash. I'll wait for my girlfriend to come back and help me. I'll feed the mosquitoes at the same time."

"Where's your cell phone? I called you for a long time but you didn't answer."

"Who throws away garbage and holds a cell phone?"

Lao Lin was very confident, as if making a mistake was normal. My heart sank again. Lao Lin really had no memory of it at all.

It is normal for people with Alzheimer's disease to forget recent events and have trouble planning upcoming activities.

It's not like she hasn't encountered similar situations before.

But now, under this tree, facing her father's puzzled face, she felt only deep helplessness.

Humans are too vulnerable to disease and there is nothing we can do about it.

Looking at his father, Lin Chaoxi forced himself to smile and pretend that nothing had happened.

"Master Lin, have you prepared the meal? I am starving."

"Have I forgotten something again?" The old man was very smart.

"Yeah, guess what you forgot?"

Lin Chaoxi pulled her up and pushed him into the door.

Lao Lin remained silent until he entered the house.

But as soon as the door opened, he shouted "Oh shit!" and ran straight to the kitchen.

There was a slight burnt smell in the air, but Lin Chaoxi ignored it.

She looked around the house and saw her father's phone on the shoe cabinet. She quietly walked over to unlock it and saw the WeChat interface, which showed the invitation she sent and Lao Lin's reply.

"Hey, I turned off the fire. Is he really suffering from senility?" Old Lin breathed a sigh of relief in the kitchen.

Looking at his father's dim figure busy and happy in the kitchen, Lin Chaoxi made up his mind and deleted the entire conversation.

"What are we having for dinner today?" she shouted, putting down her phone.

…

She didn't know what she was thinking at the time. Anyway, she deleted the WeChat conversation for 58 minutes before her father found out.

Lao Lin had been suspicious that she was not telling the truth. After dinner, while she was washing the dishes, Lao Lin went straight into the study and opened the computer version of WeChat.

When she came out after washing the dishes, Lao Lin was sitting on the sofa in the living room. He didn't turn on the lights, his face was livid, and he didn't say a word.

The air smells bad.

As far as she could remember, the last time Lao Lin acted like this was when she was in high school and she had secretly chosen the liberal arts.

Lin Chaoxi's first reaction was to run, and her second reaction was not to run. She had read too many materials related to Alzheimer's disease, and emotional instability was one of the problems brought about by this disease.

She turned around, wiped her hands on her apron, walked towards the sofa, and asked cautiously, "Dad?"

No response.

She caught a glimpse of the vaguely visible computer version of WeChat on the screen in the study and instantly understood what was going on.

She squatted down and quickly admitted her mistake: "I'm sorry, I just thought that you would be unhappy if you saw that one, so it doesn't matter whether you read it or not..."

"Do you think your father is losing his mind?" Lao Lin asked in a cold tone.

"No, no..."

"Then why do you think your father would get angry because of his daughter's little kindness?"

Lin Zhaoxi raised his head slightly, very surprised: "Eh... why is that?"

"You went on a blind date?"

She was stunned. At that moment she had guessed countless reasons, but she never thought it was because of this.

Lao Lin's expression is still very ugly. Your mind is unpredictable.

"How do you know that?"

"Haha, to be honest, the Dean of Academic Affairs of your internship school is my classmate. He learned about my illness from my daughter and came to show concern for me. At the same time, she further reported to me that the family of your blind date was very merciful and did not take my illness into consideration. They decided to continue to include you in their son's harem. In parentheses, you were downgraded to one of the candidates. I hope we can continue to make great progress."

“…”

"What on earth are you thinking about?" Lao Lin's tone was unusually stern.

"I'm at the age where I should find a boyfriend."

For some unknown reason, Lin Chaoxi didn't explain to Lao Lin. In fact, she didn't know it was a blind date party before entering the cafe.

"As if? What exactly is your life plan?"

"Be a teacher." Lin Chaoxi replied.

"Oh? So you went on blind dates because you really wanted to have a boyfriend, and you became a teacher because you really liked students?"

Lin Chaoxi couldn't give a direct answer. She said, "Comrade Lin, you should know better than anyone that you can't do whatever you like in this world."

For example, you like math, but you end up giving up your studies.

I like Pei Zhi, but in the end I can only sigh in despair.

Isn't growing up about learning to compromise with the world?

After she said that, Lao Lin didn't say anything else.

Lao Lin just looked at her deeply.

He had seen the same look in Lao Lin's eyes when she gave up studying Mathematical Olympiad; he had also seen the same look in Lao Lin's eyes when she and he argued about the division between arts and science.

He was disappointed with her decision, but he resisted the urge to influence her life.

People must be responsible for their own decisions. This is Lao Lin’s life creed.

Lin Chaoxi took a deep breath and stood up by holding his knees.

"Have you ever thought about this..." Lao Lin did not give up this time and continued to persuade, "Ten or twenty years later, I will be lying on the bed dying. When you think of today's conversation in front of my bed, what will you want to say?"

"Dad, what's our bank card password?"

"215,000."

After answering, Lao Lin patted the sofa and warned her: "Comrade Lin Zhaoxi, don't kill the conversation casually."

Lin Zhaoxi: “…….”

"Think about it carefully. Will you regret not pursuing your dreams when you were young enough to do so?"

Lin Chaoxi stood in front of the sofa. There was little light in the room and the walls in the distance were dim.

Dream, what is her dream?

The chicken soup that Lao Lin had been making was very fragrant. She couldn't help but recall Pei Zhi's question and the frustration she had when she wanted to answer it but had to give up because she didn't have the ability.

It should be regret...

She looked at Lao Lin and said, "There is a question."

Ten minutes later.

Lao Lin put down the pencil he was using to calculate and asked, "It was Pei Zhi who came up with it, right?"

“…”

Lao Lin sighed: "So why are you dating? There are such good boys out there, but you don't chase them."

Lin Zhaoxi: "I also like the French shark. Can I catch up with it?"

Lao Lin: “Who is that?”

Lin Zhaoxi: "Magneto."

Lao Lin: "What a coincidence, I like it too."

“…”

"So, if you solve this problem, Pei Zhi will agree to be your boyfriend?"

"Can you please stop being so obsessed with your brain? This is the entry question for joining the modeling competition team."

"I'm afraid not." Lao Lin's tone was very firm.

"I do not understand."

"This is a stone thrown to ask for directions." Lao Lin took off his reading glasses and looked far-sightedly, "It is not asking about the direction of the modeling competition, but the p/np problem."

Lin Zhaoxi looked at the question again and finally understood where the feeling of being scratched by scales came from.

The p/np problem is one of the Millennium Problems, with a million-dollar prize, and is on par with the Riemann hypothesis and the Pomchartrain conjecture.

Pei Zhi is really...

"Very ambitious." Old Lin made the final decision and was very pleased: "I corrected myself. This should not be the question asked by Pei Zhi. This is the intention of the tutor. Is the professor who studies Hamiltonian circuits in your school's mathematics department Pei Zhi's tutor? Is his surname Shen or Zeng?"

Lin Zhaoxi was speechless: "You know too much."

"Not much, not much. I also know that although this problem is still a long way from truly solving the P/NP problem, it is worthy of respect for the courage to solve it and rewrite the course of human history."

"Yeah, that's right." Lin Chaoxi thought, what courage, it's really enviable.

"Then do you want to apply for the graduate program in the mathematics department of your school?" Lao Lin changed the subject.

Lin Chaoxi looked at him stupidly, not knowing how the topic had come to this.

"Prove that P=NP, greatly improve the speed of computer operations, and the time to conquer countless diseases, including Alzheimer's, will definitely be greatly shortened..." Lao Lin continued with emotion, "Why don't you consider changing your father's major?"

Lin Zhaoxi pointed at himself, then at the question on the table: "Boss, I can't even understand it, okay? There is a gap of four years of professional study between me and the postgraduate entrance examination in mathematics!"

Lao Lin smiled and said earnestly: "You feel that you can't catch up with Pei Zhi, but is the distance between you and him greater than the distance between him and the P/NP problem?"

It is simply irrefutable.

Lin Chaoxi turned around and walked towards his door.

Lao Lin was still laughing, and his low, hoarse laugh echoed behind her.

"Ms. Lin Zhaoxi, in this long and wonderful life, if you really find what you want to do, then it is never too late to start again no matter when you decide to do it."

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P refers to problems that can be solved quickly by computers, and NP refers to problems for which we want to find the best solution. If P = NP, then we will easily find the solution to any given problem. P = NP means that society as we know it will undergo a huge change, and the degree of automation in medicine, science, entertainment and all tasks in human society will immediately undergo a qualitative leap.

[US] Lance Fortnow. The border between possible and impossible: an interesting history of the PNP problem