Introduction to Advanced Mathematics

Copywriter: If I had known, I shouldn't have bought that eraser in the first place.

***

A strange thing happened recently. Every midnight, a voice whispered in my ear, "Do you have an eraser?"

At first I thought it was an auditory hallucination, but the sound kept on keeping me awake.

"Do you have an eraser? Do you have an eraser? Do you have an eraser?"

It didn't stop until I answered "no".

The next day it came again, "Do you have an eraser? Do you have an eraser? Do you have an eraser?"

"I said no."

Day three, "Do you have an eraser? Do you have an eraser? Do you have an eraser?"

"No! Go away!"

On the seventh day, I became furious and asked viciously: "Are you a repeater? Are you a repeater? Are you a repeater?"

The sound stopped.

After a pause of five seconds, he asked again cautiously, "Do you have an eraser?"

"..." I sighed.

On the eighth day, I went to the stationery store to buy a new eraser. As soon as it appeared that night, I rushed to say:

"I have an eraser, then what?"

"Great!" The voice sounded happily, "I'll take you to a place!"

In this way, I followed a voice, walked into the darkness, then stepped into the light, and met the owner of the voice.

"My name is Ling Hun."

He is translucent, and I can clearly see the bookshelf behind him through him.

"You look like a ghost."

"I'm a ghost!" he admitted readily.

"...I really saw a ghost."

"You are!"

I looked around and found that this is a place that looks like a "well", the mouth of the well is so high that you can't see the edge.

The well is not made of stones, but circles of bookshelves spiraling up. The bookshelves are filled with densely packed books. You can't imagine how many books there are. My density phobia is about to commit.

"This is where?"

"This is the memory library, where memories are stored."

"Oh...and then what?"

"I want to ask you a favor!"

Ling Hun flew around, took down a book, and spread out a page in front of me.

"See my name?"

In the middle of that page is clearly written "Linghun", above and below it, there are many other names.

"Saw."

"Please help me wipe it off!"

"Why?" I don't like doing things for people without asking why.

"Because you have an eraser!" Ling Hun replied innocently.

"..."

"I'm asking you, what is this book? Why did you erase your name? What happens after erasing?"

Ling Hun was overwhelmed by my series of pressing questions: "Just... let me ask you a favor...you have so many questions..."

"..."

I tried my best to restrain myself, not to be angry with a ghost.

"Forget it, let's ask questions one by one. Since you are a ghost, it means you are dead, so how did you die?"

"Leukemia." He answered quite simply this time, "I'm a sick ghost."

"What book are you holding?"

"It's my dad's memory book. The names on it are all people my dad knows. Look, this is my grandpa, this is my grandma, and this is my uncle..."

"Stop!" I interrupted his lengthy genealogy introduction, "So? You asked me to erase your name on this, can you still erase your name?"

"Yes!" He nodded desperately, "After erasing it, my father won't remember me, as if this person never existed in his life."

"You want your father to forget you, what's the reason?"

Ling Hun lowered his head: "After I died, my father was very sad. He didn't smoke at first, but now he often smokes on the balcony for a whole day. I went to see him a few times, and his hair was all gray, so old that I Almost unrecognizable. I don't want to see him depressed like this again, please let him forget me, if he forgets me, he won't be sad anymore. "

"Are you sick?" I scolded him.

"I have, leukemia."

"..." I, "I didn't mean this! Isn't it normal for your father to be sad after your death? There is nothing wrong with him being depressed now, but sadness will always be healed by time. When he comes out one day, at least he can Thinking of those happy memories when you were alive. Don’t you think it’s more cruel to erase his memory willfully?”

Ling Hun listened to my words, pouted for a while, and still insisted: "I don't care, I just want to wipe."

"I won't help you." I regretted buying the eraser.

"If you don't wipe it, I will, I will...I will cry for you!"

I was terrified: "You are not allowed to cry! I heard that if you see a ghost shed a tear, you will have nightmares for a whole year!"

"If you don't help me, I'm going to cry!"

Ling Hun sniffed vigorously to build up his emotions, but he couldn't shed a single tear.

"Hey, it's so annoying. There's nothing sad right now, so I can't cry." He said bitterly.

But he didn't give up threatening me: "But I will cry one day, if you don't help me today, I will keep pestering you until I cry."

"How can you be so unreasonable?"

"I'm such an unreasonable ghost."

I was so annoyed by him that I couldn't help it, and I thought to myself that I didn't know him, and I didn't know his father, and his father forgot what he had to do with me.I just want to get rid of him quickly and get back to sleep.

"Okay, okay, bring it here, I'll help you." I said angrily.

After hearing this, Ling Hun quickly turned over the page just now and handed it to me, for fear that I would regret it.

My hand holding the eraser stopped at the line with his name: "Are you sure you want to erase it?"

Ling Hun bit his lower lip and nodded.

"I can't write after I wipe it. Don't regret it in two days and come to me again and ask me, 'Do you have an automatic pen? Do you have an automatic pen? Do you have an automatic pen?'"

Ling Hun was so teased by me that he burst out laughing, he couldn't cry, but when he laughed, the corners of his eyes glowed faintly with silver light.

"I'm sure, you can do it."

I shook my head helplessly, picked up the eraser and daubed on the book.

"Be careful," he brought his little head closer to supervise the work, "don't rub anyone else."

I stopped the movement of my hand: "You blocked me."

"..." He withdrew back slightly aggrieved, "I'm so transparent, can I still block you..."

I wiped it hard for a long time, and then I saw that Ling Hun's name had only faded a little.

"Are you sure this can be wiped off?" I doubted.

"The stronger the memory, the harder it is to erase it." He pointed to the name that showed signs of fading. "But look, hasn't it faded? As long as you persist, you can always erase it."

I have no choice but to continue, but I have to admit that what Ling Hun said is right, his name became more and more shallow under my persistent homework, and finally disappeared completely.

After wiping, I stared blankly at the blank line, Ling Hun's father, probably won't go to the balcony to smoke tomorrow, right?

"It's erased, here you are." I returned the book to him in a rather low voice, who knew that he handed over another book.

"This one belongs to my mother!"

I:? ? ?

"I don't think you are a dead ghost, but a fool. You want your mother to forget you?"

Ling Hun lowered his head: "After I died, my mother was very sad. She locked herself in the bedroom every day and washed her face with tears. I went to see her a few times, and the corners of her eyes were wrinkled. She used to be the most beautiful. I don't want to see her depressed like this again, please let her forget me, if she forgets me, she won't be sad anymore."

"You, you unfilial son!" I scolded him.

"Death is the greatest unfilial piety. Is there any greater unfilial piety than death?"

I couldn't answer, so I copied the notebook after a while. Anyway, he insisted that if I buy one get one free, I'm doing good deeds.

This time it took more effort to erase than before, the corners of the eraser were all bald, and it was only then that Ling Hun was barely erased from his mother's memory book.

"Thank you, you are so kind." Ling Hun thanked me while handing over another book.

I:……

"This is my brother's, kid, I don't remember very well, it's easy to clean."

Seeing that I didn't pick up for a long time, just staring at him with a gloomy expression, Ling Hun smiled carefully and said: "He is only six years old this year, you don't want him to live his life with the memory of his brother gone, childhood, it's best to Carefree."

I don't want to argue with Ling Hun anymore, because I'm really sleepy.I took it numbly, and erased his name without any waves in my heart.Ling Hun said that children don't remember deeply, I don't think so, at least my wrists are still quite sore after wiping.

Then I saw Ling Hun still holding a book in his hand without any surprise.

"How many people are there in your family?"

"Only five," Ling Hun stretched out five fingers, then withdrew his thumb, "Without me, it would be four."

Since it was the last one, I simply sent the ghost to the east, and stretched out my hand in a broken pot: "Bring it."

Ling Hun handed it a little cautiously this time: "This is the brochure of my family's rhubarb. I raised the rhubarb from such a small one, and it is very obedient."

"……dog?"

"Well," Ling Hun nodded embarrassingly, "German shepherd and native dog are of mixed blood."

"..."

Ling Hun lowered his head, and before he could make a speech, I robbed him and said, "Yes, yes, I understand. After you died, Rhubarb was very sad. He didn't eat or drink every day, and stood by the door waiting for you to come back. wrong?"

Ling Hun pushed me coquettishly: "Look at you, you have learned to answer quickly."

I grabbed Rhubarb's book expressionlessly, opened it for a glance, broke down, and threw it back to him.

"look by youself!"

Ling Hun was puzzled, opened it, and saw that every line inside read, "Woof woof, woof woof, woof woof..."

"Haha, aha," Ling Hun said dryly, "This one is really hard to find..."

"nonsense!"

Ling Hun rubbed the pages of the book reluctantly: "Then let Da Huang remember me for now. Unexpectedly, the one who can't let go is you..."

"Are you all right?" I didn't want to disturb his nostalgia, so I yawned, "I want to sleep."

"Don't sleep!" Ling Hun flew over to shake me, but unfortunately he couldn't touch me, "You will get out from here when you fall asleep!"

"It's not over yet? Didn't you say that there are only five people in your family...and a dog?"

"There are a few more people who are very important, please wipe them for me." He held a stack of books pitifully, "Please, just help me with them."

I'm going crazy: "Who are these people?"

"These are...the neighbor next door, Uncle Ji, the next door neighbor, Aunt Yang, and..."

"Enough!" I yelled, "Why should I even take care of the memories of your neighbor next door? There are so many people in the world who know you, should I wipe them one by one?"

"Also, is this broken library of yours so unmodernized? Doesn't it have an electronic entry? Can you just press Ctrl+F for full-text search and then Shift+Delete to fix it?"

Ling Hun was frightened by the maddened me, holding the book and muttering: "No, it doesn't have to be all, just, just these few..."

I calmed down a bit, seeing his pitiful appearance, the hell I couldn't bear it again.

"Okay, there are only a few books! No more books!"

Ling Hun nodded like pounding garlic.

I waved helplessly, never thinking that one day I would be helpless with a ghost.

Ling Hun immediately grinned happily, I really admired his emotional change.

I picked up the eraser resignedly: "Uncle Ji next door, get it done.

"Auntie Yang, the neighbor next door, get it done."

"What's next?" I held out my hand.

"This is the last one." He handed it over with hesitation. "His name is Ji Yi, and he is my boyfriend."

"What are you?"

"My boyfriend." Ling Hun put the book heavily in front of me as if he had made a lot of determination.

I have no intention of judging his personal emotions.

"You are willing to do this?"

"If you can't bear it, you have to be willing." Ling Hun sighed, "Whoever makes him the most important person to me, I can't bear to see him sad."

"Isn't it good to live with your memory?"

"He loves me so much, he can't live without me."

"So confident?"

"Forget about me, wouldn't it be good to find another person who loves each other and can still be blessed?" His voice was inexplicably sad, "There are so many outstanding souls in the world, why bother to cling to one soul."

"You're such a selfish man." That's all I could say.

"Aren't you sleepy?" he begged me. "I swear this is the last one."

I had the illusion that I was the ultimate villain of the novel, and without a word, I began to clean up Ji Yi's memory about Ling Hun.But this time I wiped it for a long time, but Ling Hun's name didn't show any signs of fading.

"I feel like this is indelible."

"How is it possible?" Ling Hun walked around and looked left and right, trying to find a little bit of possible gray in the black letters.

"Try again, maybe it's just too unforgettable."

"Tch," I sneered and continued to wipe, "You actually asked me to erase an unforgettable relationship with my own hands, are you a devil?"

"The devil isn't much," he muttered under his breath, "just a ghost."

After a quarter of an hour, I threw the eraser on the ground in annoyance.

"No, I can't wipe it off."

"It shouldn't be," Ling Hun looked anxious, "Why can't it be wiped off?"

He had a flash of inspiration: "I have seen the manual of this museum, just wait for me!"

Ling Hun found the manual and looked through it with me.

"...here! Explanation about the indelibility of names, page nine!"

I immediately turned to page nine and found the article, which read:

——If B’s name cannot be erased from A’s memory book, it means that A is the culprit who killed B.

——If you want to erase, unless B forgives A's behavior.

I stared at Ling Hun, and he also stared blankly at this line of words. After a while, he shook his head and took two steps back.

"Impossible. It's impossible." He shook his head.

I didn't miss any expression on his face: "You didn't die of illness. You were killed."

"Ji Yi can't kill me!"

I handed Ji Yi's book to him: "Then you come?"

"..."

Ling Hun hugged his knees and squatted down as if drained of strength. He was a cheerful and optimistic ghost not long ago, but now he looks like a dead ghost.

"I did not die of illness, I committed suicide.

"Ji Yi and I have been neighbors since we were young. We have a very good relationship. His parents treat me like their own son, and my parents treat him too.

"We grew up together, and when we reached adolescence, we found out that we had a crush on each other, and we struggled for a while, and then followed our instincts to be together.

"Both of our parents are quite conservative, so naturally we didn't tell the family about this kind of thing, until one day my younger brother pushed the door directly into my bedroom.

"At that time, the parents of the two families were playing cards next door. My brother ran over and said that my brother and brother Yi were playing kiss, and then the volcano erupted.

"The two families did not agree with us being together, I was locked up, and I heard that Ji Yi was going to be sent abroad by my family.

"Later, we both tried our best to escape, but there was nowhere to go. One day on the street, I was very desperate, so I said to Ji Yi in a twitch, why don't we commit suicide.

"Now you know why I asked you to erase the memory of my family. After I died, they all felt that it was their own fault. I can accept that they live sadly and miss alive, but I don't want them to live with guilt, obviously It's just me being stupid."

"You're pretty stupid." I commented angrily.

Ling Hun had a pitiful expression: "I have already paid the price with my life for my stupidity, so why don't you make it worse?"

I rolled my eyes to the side in displeasure: "Then what? He killed you, and he doesn't want to die anymore?"

"Of course not! The two of us took medicine together, only I died, and he was rescued back."

"Then how do you explain that he killed you?"

"I don't know!" Ling Hun shouted very unhappy, "You just don't believe that Ji Yi is sincere to me! You are a conspiracy theory!"

"Ha!" I laughed angrily.

"Ji Yi treated me so well. After we made an appointment to die together, we got two bottles of medicine in the dark. One for me, one for him, one for me, one for him. There are too many medicines. Halfway through, I was tired, and if I asked him to feed me like a baby, he would feed me one, and myself one..."

"...You're too lazy!!"

There was an eerie silence for half a minute, and Ling Hun and I shouted in unison: "Feed the medicine!"

Poor Ji Yi, he should be the most innocent murderer in history.

Ling Hun found out the truth and was filled with grief and joy.

"It's because of this, no wonder it can't be wiped off, so what should I do now?"

"What do you say? It says, unless you forgive him."

"But I don't blame him at all."

Ling Hun raised his head, and shouted at the mouth of the well, which was beyond his sight: "I forgive Ji Yi!"

The echoes rippling away layer by layer, I forgive Ji Yi——I forgive Ji Yi——I forgive Ji Yi——

"And now?" he asked.

I picked up Ji Yi's book and tried it, but it still didn't work.

"It's impossible for me to go directly to Ji Yi, he can't hear me. I tried so many people, only you can hear me."

"Then do you think if I go to him and tell him, will it be regarded as a mental illness?"

"Then what should we do? This solution condition is simply a paradox."

Once again things were deadlocked, we both fell to the ground exhausted, especially me, I was sleepy and crazy.

"Can you come up with a solution? If I can't figure it out, I'm really going to fall asleep."

My eyelids became heavier and heavier, and just as I was about to fall asleep, I was awakened by a loud cry.

"Ah! I thought of it!" Ling Hun sat up excitedly, "If I don't remember him, naturally I won't blame him, then it's tantamount to forgiving him!"

My mind was still at a standstill: "What do you mean?"

He turned out the book with his name written on the cover, and pushed it in front of me: "You can erase my name after you erase his name, can't you?"

"Are you stupid? You won't even have his memory."

"I'm a ghost, what's the use of remembering?"

"You are a ghost, you only have memory left!"

He stared at me blankly, and suddenly tears flashed in his eyes, and big tears rolled down.

"Damn it! Don't cry! I'm afraid of you, can't I just do it?!"

I searched all over the place for the eraser that I had thrown away, and complained in my mouth: "I'm out of my mind, why should I buy an eraser..."

I found the eraser, I didn't dare to look at him, I squatted on the ground and tried my best to erase Ji Yi's name on Linghun's book.

It's really hard to wipe, I feel that the paper is almost worn out by me, and the words are only a little faded.

"The last one belongs to him, don't forget! Promise me! Be sure to erase my name too!"

There was a deep cry in his voice, and I sped up my movements in panic.

"Don't cry, I'll just wipe it off. Look, it'll be quick."

A drop of water fell on the paper, and I couldn't help raising my eyes. I regretted it when I raised my eyes. Ling Hun stood there, tears falling down like broken pearls.

I saw more than a drop of tears from a ghost this day. Will this nightmare of mine survive into the next century?

"Promise me!" he cried.

"...I promise you." I said without looking up.

I have worked hard for a long time, and the eraser has worn away more than half, and finally there is no each other's name on the two books.

There is no Ling Hun in Ji Yi's book, and there is no Ji Yi in Ling Hun's book.

Even the unforgettable feelings are not worth half an eraser.

I'm freed from my fossilized state: "It's over."

There were still tears on Ling Hun's face, but there was no sad expression on his face anymore.

"Thanks for your hard work." He changed back to the optimistic ghost when we first met, "here is the last one."

I:? ? ?

"But you clearly said that it was the last one just now!"

"When did I say that?"

"Just when I wiped Ji Yi's name!"

"Who is Ji Yi?" He asked blankly.

"..." I said, "It doesn't matter who it is, just bring it."

This time, it was very easy to wipe off. I wiped it lightly and it fell off. I guess I am not someone who is very familiar with him.

"This time it's really over, thank you!"

I waved my hand and yawned again. My mind was getting heavier and heavier, and I was about to fall asleep.

In the dimness, I vaguely heard another voice.

"You don't have nightmares."

I woke up with the morning light, stretched my waist, and rubbed my sleepy eyes.

There seemed to be something, I spread my hand and found half an eraser in my palm.

I looked left and right, and couldn't figure out where it came from.

It's strange, why am I holding half an eraser?