I was guided by an adult maid to a room where there was a child in it. She was a girl with black hair. Her body seemed weak. I supposed she didn't learn any martial arts.

The maid was probably busy and that was why left immediately. I didn't get to ask, but I guessed the girl was that Gouk's daughter.

She sat on a chair in front of a desk. While leaning against it, she was closing her eyes quietly. On the desk, there were wooden plates and ink pots. A lamp with a wick in an oil plan was illuminating the girl's face.

Looking at her appearance, it seemed that she was about the same age as me. Well, since she was a cousin, I knew her name and age.

This girl was a year younger than me and her name was Syamu.

"Hi, good evening." (Yuri)

When I said that…

"…" (Syamu)

There was no reply.

'What's with this girl?'

Even if I talked to her, she didn't move her eyebrows. She still leaned on the chair and shut her eyes.

I knocked when I entered, and since the maid lightly introduced me, she was probably not sleeping.

'Is she dead?'

I became uneasy. It would be bad if she died.

I would be made the culprit. Did it mean that there was a plot of someone who try to turn me into a murderer?

I nervously came closer, and touched her face with my hand. Then, she opened her eyes.

"That's rude, isn't it?" (Syamu)

She was alive.

"Are your ears bad?" (Yuri)

When I asked that, the girl glared at me with suspicious eyes. What was she going to say with that kind of eyes?

"…My ears are fine." (Syamu)

"You're supposed to return the greetings. Well, if your ears aren't bad." (Yuri)

Even in this country, the greetings must be returned. If it was about being rude, it was the other side, not me.

I sat on a nearby chair without permission.

"Were you thinking about something?" (Yuri)

I asked so.

Well, since her father was going to war from tomorrow, she might be worried about him.

"Yes." (Syamu)

"Are you angry because I interrupt your thought?" (Yuri)

"No, you don't have to worry about that. My mind is not calming down, anyway." (Syamu)

"Really?" (Yuri)

I could heard noises and slight clamor from outside the door. It was quite obvious because the feast was held in the middle of the mansion. Before that, I probably should worry about Father.

"What were you thinking about?" (Yuri)

"You will not understand anyway." (Syamu)

That was a short reply.

"I guess so. But, I wouldn't know unless you say it." (Yuri)

"That's true. But, you'll end up in vain later." (Syamu)

'What a cheeky brat. Isn't this interesting?'

"I have nothing to talk about anyway. I'm not sure what are you thinking either, but whether I can understand or not, you can try me. Well, if it's something secret, I won't ask, alright." (Yuri)

"I don't understand what you mean by trying. Please leave me immediately." (Syamu)

"I can't do that. Well, just think of this as some sort of play." (Yuri)

Syamu let out a sigh.

'A monkey doesn't understand civilization, but it rudely enters my room, and now I have to speak to the monkey. I wonder if I should chase it away.'

That was the feeling I had from that sigh…

'Th-this…'

"If I talk, you'll go out, alright?" (Syamu)

"Yeah, I promise." (Yuri)

"Is that so?" (Syamu)

She sighed again, but she began to talk.

"I was wondering whether there are infinite Sosuu." (Syamu)

I couldn't understand what Sosuu meant for a moment.

However, as for the word 'number' was the essence of Shan language, it was combined word that meant to be united, so I was able to understand somehow. It was a prime number.

"Are you talking about, 2, 3, 5 and so on?" (Yuri)

"…Yes." (Syamu)

"Then, it will be 11, 13 and 17, right." (Yuri)

"That's what I'm trying to say." (Syamu)

It seemed that she was talking about prime numbers after all.

'What's with this girl? You're still six years old. She is certainly one year younger than me. Then, she is supposed to be six years old, right? She has the nerve to be so direct, but she is bright, huh? Doesn't six years old should be saying something like 'Okasan, I want this–…' while playing toy car, or something like that? Does that mean that she is a true Ojou-chan?'

She was really amazing as if she had no problem taking a private elementary school exam.

"Are you talking about prime numbers?" (Yuri)

"As I said before, I was thinking whether the prime number is infinite or not." (Syamu)

'Oops, this girl is…'

"I was wondering what you were thinking about." (Yuri)

It was a mystery why children of this age cared about such things.

'Moreover, that is… what… there are other things you can do. I just don't get it. Listen to me, just play house or something.'

"…Haa, as I expected, you don't understand it. Please go out." (Syamu)

"I understand, you know." (Yuri)

Prime numbers were infinite.

I didn't have the proof, but I just knew it.

"Haa, fine, please tell me about it." (Syamu)

I was looked down.

She probably thought that I was bluffing.

"By any chance, are you still unable to prove it?" (Yuri)

"I know there is a proof." (Syamu)

Nonetheless, there was a proof. It was a proof from the BC era on the Earth. Speaking about that, it was obvious. I supposed there were genius at the level of Euclid in this world too.  I was deeply impressed.

But the, why was this girl thinking of something that had already been proven? Did she like to think about number theory?

"As expected, you can't explain it." (Syamu)

"I can." (Yuri)

"Then, explain it immediately." (Syamu)

I was somehow getting annoyed. She was probably judging that I was trying to buy a useless amount of time.

'Hmm, how should I explain this? It is written in my book at home.'

"Let me think." (Yuri)

"…Well, go ahead. It would be pointless anyway." (Syamu)

I slightly ignored her, and began to lost in deep thought. I wrote in the book over a year ago, but as I expected, I quickly remembered because my brain was young. I remembered the proof as if I pulled a string.

"Let N be the number of two or more. Then, N and N+1 don't have a common divisor other than 1 for each number." (Yuri)

Since 'N' wasn't in the Shan language, it was convenient.

"…?" (Syamu)

She looked at me with a somewhat puzzled face.

"Do you get it? Since the difference between N and N+1 is 1, it would be strange if there were 2 or more common divisors." (Yuri)

"Well, yes." (Syamu)

"Then, if we multiply N and N+1, that number has two or more prime numbers as factors. It is not possible to think that the prime numbers overlap like 4 and 8. Then, let M be not a prime number. If we multiply M and M+1, then the number has more than 3 prime numbers as factors. And since it can continue infinitely, prime numbers are also found infinitely. Thus, prime numbers exist indefinitely." (Yuri)

When I finished saying so, Syamu absentmindedly opened her mouth. For quite some time too…

"…Ah, uhmm… uwah, but…" (Syamu)

That was what she said.

She smoothly wrote on the wooden board at hand, and it seemed she confirmed it after a while. It was a simple proof that there was nothing wrong with it, so it should be wrong.

If I got wrong with this, I might shut myself in a hole as if I was having seizure, or I might want to fall from high place and commit suicide.

Sometime later…

"That's amazing." (Syamu)

And Syamu muttered so…

She looked at me with different eyes.

'How about that? Have you given up now?'

"Did you read books of the House?" (Syamu)

Her face turned bright as if she had a complete change. It was because I was grinning.

"Well, yeah." (Yuri)

"I'll remember it well. Thank you very much." (Syamu)

"It is a simple proof. I can do this much." (Yuri)

"If you don't mind, can you lend me that book?" (Syamu)

Her eyes were s.h.i.+ning brightly.

'No, whether it is a book or not… How should I say it… the book is written in j.a.panese. Since it has different character system, maybe no one else can read it.'

"Sorry, it's not a book I read at home. I read it in the capital." (Yuri)

I lied to her in the spur of the moment.

"In the capital, is it? I see…" (Syamu)

Well, if she when to the capital, that was something to be expected. I didn't know whether the solution was similar or not.

"Uhm, what is your name?" (Syamu)

"Yuri." (Yuri)

"My name is Syamu. Do you come from the capital, Yuri-san?" (Syamu)

"No, I'm from the neighborhood area." (Yuri)

"Neighborhood is it? Are you my relative?" (Syamu)

"You are my cousin." (Yuri)

"When you say cousin, are you Rook-san's?" (Syamu)

"Yes, I am." (Yuri)

It seemed she knew the name of Rook.

"I see. You're not from a Knight family." (Syamu)

Well, a Knight family was a Knight family. Since we were relatives to a General household, if the head of the family had a qualification of a Knight, the whole family would be recognized as a Knight family.

"I'm jealous." (Syamu)

'What are you jealous about? You are living better than I am.'

"My family doesn't understand me much, and they aren't buying me a lot of books." (Syamu)

She probably thought that I was educated because I read a lot of books. It was probably reasonable to think so. Actually, it was only a memory. Today was the first time I heard the word of prime number.

Syamu made a shabby face.

"Is that so? That's a shame. You're so smart." (Yuri)

"Eh… that… is…" (Syamu)

Syamu's face was reddened.

"Do you really think so?" (Syamu)

"Well. However, if everyone is as smart as you, I will be an inferior student immediately, and you'll be far ahead from me." (Yuri)

"That's not true. You are better." (Syamu)

'It is nice to be complimented, but rather than you who only live for six years, I have lived five times more than you, so it can't be helped to be inferior. That is indeed sad.'

"I appreciate that." (Yuri)

"If you don't mind, please teach me more." (Syamu)

"Even if you say that, what are you interested in?" (Yuri)

"I'm interested in everything." (Syamu)

"Everything? That's impossible." (Yuri)

"Ah, maybe not all. How about your Otou-sama, or Togi, or maybe knitting and embroidery…" (Syamu)

'That's pitiful, Gouk. Even though you have good-looking daughter, you are not interested in her hobby at all. Because of that, you are not buying her books.'

This girl was smart. It should be fine if she attacked him by acting like a spoiled child. I was convinced that she could ask the father to buy books or learn Togi.

"Nevertheless, I am also not very knowledgeable. The only thing I can teach you maybe mathematics." (Yuri)

Perhaps I should say that it was right to say since mathematic was the only thing I could teach. It was obvious that I had come to another world, and the laws of physics also might have changed. Even if I taught with Chemistry with a triumphant face, the laws were completely different, and it was normal if they didn't apply in this world.

"Of course, it doesn't matter. Please take care of me. Let's talk about various things." (Syamu)