The Birth of Little Sister Yoo A-rin
Seol-ae was cradling A-rin, soothing her, completely absorbed. Jin-hu was rummaging through drawers next to her, searching for diapers and moisturizing cream and such.
Now able to walk freely, I lay in my crib, gazing at A-rin.
She's so tiny. Her eyes can't even open properly, and her cries are abnormally loud for her lung capacity. Was I like that twenty months ago?
Good. Now I'll be able to move around even more freely.
Practical thoughts followed right after.
From now on, Seol-ae and Jin-hu's supervision would loosen up significantly.
Up until now, I'd been a good kid who never put toys in my mouth or dropped things.
Since I'd been such a well-behaved child who never caused any big trouble, Seol-ae and Jin-hu would feel safe pouring all their energy into taking care of A-rin.
Seize the chance to slip into the study and get the research materials.
That's what I thought at first. That's what I tried to think.
But then...
"Waaah!"
As A-rin burst into tears, Seol-ae stroked her with a tired smile on her face.
"It's okay, A-rin. Mommy's right here."
Jin-hu was busy gathering things for A-rin beside her.
Watching that scene, a certain memory suddenly surfaced.
-Becoming a mom is just as valuable—no, even more so—than doing math.
That's right. I'd said that myself.
When Seol-ae chose childcare over research, I'd told her it wasn't the wrong choice and even cheered her on.
And yet.
...What the hell am I thinking right now?
On the first night A-rin had come home to this world, my very first thought was 'Their supervision will loosen up.'
Our family had grown by one. A little sister had been born. Mom and Dad were so happy—and so exhausted.
And here I was, thinking about my paper.
...After everything I said myself.
Building a family is more valuable than doing math, I'd proclaimed.
But I, the one who'd said it, hadn't understood at all.
"M-mommy..."
I closed my eyes without a sound, quietly reflecting in remorse.
I didn't need to rush. The Riemann Hypothesis had stumped everyone for 160 years anyway.
So... I can wait just a little longer, right?
Obsessing over the paper to the point of neglecting my family was just my own selfish desire. I even remembered Jin-hu chewing me out in my past life for being obsessed only with results.
Now that I'd been reborn, I wanted to live a life where I could look at myself and those around me.
After A-rin was born, I changed a little.
I hadn't given up on writing the paper. It had just slipped a bit lower in my priorities. It was a long-term battle that would take years anyway.
And I decided to accept my identity not as the reincarnation of Shin Ijun, but simply as Yoo A-yeon, a little girl with the memories of Professor Shin Ijun from his lifetime.
I'd been hesitating before, but having a little sister seemed to fully trigger it.
After Mom put A-rin to sleep and slumped on the sofa in a daze, I quietly walked over and sat beside her.
"A-yeon?"
"Yeah."
I leaned my head against her just a little. I didn't throw a tantrum—that would worry Mom. I was a good kid who didn't throw tantrums.
Mom looked down at me for a moment, then stroked my hair.
Pet, pet.
"Our A-yeon's such a good girl."
"Nihihi."
When loved ones touch, oxytocin gets released. And between 12 and 36 months after birth, the brain's limbic system develops, making you super sensitive to hormone release. So... feeling all warm and fuzzy in my chest when Mom pets me is totally natural. Nothing weird about it, right? This precious oxytocin is too good to hog all to myself.
Whenever A-rin started crying, I'd go to her side and gently hold her hand. Strangely, she'd stop crying the moment I did.
"A-yeon's taking such good care of her little sister. She's a big sister now."
Dad watched us with a pleased smile.
Sister.
That word echoed strangely in my heart. Yeah. I'm a big sister. The perfect example for A-rin.
Hee-hee. My mouth curved up without me realizing.
At 24 months postpartum. Two years old. The transition from infancy to early childhood.
I'd already gotten through all the annoying baby milestones.
Now I wore panties instead of diapers, didn't drink breast milk, didn't use a pacifier, didn't cry all the time, and most importantly, I could walk steadily on my own.
My oral structure wasn't fully developed yet, so I couldn't speak fluently, but I could still communicate my intentions clearly to Mom and Dad.
That said, I didn't want to get caught sneaking into the study to look at research notes and work on the paper, so I'd been vaguely waiting until I could open the door by myself.
And finally, the chance came.
Naptime, with Mom, A-rin, and me all sleeping together. I'd stocked up on sleep the day before to build my stamina.
Snap! I opened my eyes and slipped right out from under Mom's arm.
Then I tiptoed toward the study.
"Urgh—."
Standing on tiptoes, stretching my arms high—I could barely reach the doorknob. I'd been doing stretches whenever I had the chance just to reach this thing.
Click. The study door finally opened.
I'm in...!
I quietly pushed the door open enough to slip inside without waking Mom.
Bookshelves packed with books, the woody scent of furniture, boxes piled in the corner. I started slowly searching for the materials.
I scanned the lower shelves—those seemed to be the stuff Mom had studied during her master's program.
I recommended this book to her before. Oh, this one too. She got the hardcover? That's pretty pricey.
Okay, nothing on the lower shelves. I looked up.
"S-so highhh..."
But I was a kid who didn't know the meaning of giving up. I pushed the study chair with both hands, then climbed up using my whole body.
"Hyah—!"
It wasn't easy for a child's body to climb an adult chair, but I managed it with all my might.
Worth all those daily gratitude wiggles and mental rehearsals.
Activating my semicircular canals for balance, I reached for the upper shelf and grabbed a clear file folder.
[Professor Shin Ijun's Research Materials—Research on the Riemann Hypothesis? Order unknown]
Mom's handwriting on the organized file.
Slide. Opening it revealed messy equations in my past life's handwriting.
ζ(s) = Π(p: prime) 1/(1 - p-s)
Seeing the familiar expansion of the Riemann zeta function, I was certain this was the research notebook from those three months of hell I'd endured.
"This is it...!"
But my joy at finding the research notes was short-lived. I ran into another problem.
"Huh...? Why can't I read it...?"
The handwriting was so atrocious I couldn't decipher it!
Did my past self really write like this?
Two years and ten months after death, viewing it again from a child's body—it was truly horrific penmanship.
"It could win a dance battle with an earthworm..."
Suddenly, I recalled something Mom had said over her shoulder when I was a newborn.
-No one else could interpret it, so as his only disciple, I ended up taking it over.
The handwriting was so bad no one could read it, which is why Mom—not some other professor—got the materials. And that's why I could read this notebook now.
Should I be happy about that or not...?
Anyway, if I take the time, the memories will come back slowly. I've got plenty of time.
With that thought, I secretly tucked the organized research notebook onto a low shelf in the study where I could reach it easily.
Now, if I went back to Mom's arms and pretended to sleep, it'd be perfect evidence destruction.
Then, if they asked what I was doing during that time, I could claim the whole alibi. Alibi fabrication is basic technique.
"Nihihi."
What is A-yeon doing in the study?
Lately, A-yeon had been sneaking out during our shared naptimes, slipping into the study, then coming back like nothing happened and pretending to sleep again.
She seemed like she didn't want to get caught, so I'd been pretending not to notice. But now I was curious.
What on earth was she doing in there? It was full of major textbooks, general books, classic literature—stuff too hard for a kid to read.
Just curiosity?
When I mentioned it to my husband, he pondered for a moment before replying.
"Honey. Our A-yeon's grown way faster than other babies, right? She memorizes letters and words in no time."
"Yeah, she has."
"What if our A-yeon is actually a real genius? Like top 0.01%, Einstein-level. That's why she can understand hard books."
"What? Nah, it's not that extreme. She's advanced, but still within understandable limits."
"I've got a hunch. I didn't say before, but when we named A-yeon, we got help from Ijun. Hoping she'd inherit smarts from that brainiac so our A-yeon would be super smart too. Maybe she really is."
"R-really?"
I'm not usually into superstitions, but with my husband saying it so seriously, my heart started to waver.
What do I do? At mom cafes, the ladies bragging 'Is my kid a genius?' get labeled as cringey. And here I am, feeling like one of them.
In the end, we decided to test it. With a math book way above her age level—elementary school early grades.
"A-yeon, want Mommy to teach you math?"
"Yay!"
A-yeon came running over with a big grin.
It hit me again how angelic she was. Compared to the spoiled brats I'd heard about on mom cafe stories, she never threw tantrums, never cried, loved every activity with me— I was grateful for how well she was growing up.
"This is addition, this is subtraction, this is multiplication, this is division..."
"Yeah! Yup yup!"
A-yeon nodded along attentively as I explained the four operations from the math book. She'd been the same when learning letters and words. It seemed like she was just nodding absentmindedly, but amazingly, she memorized everything after one go.
I read the math concepts aloud, then started asking applied problems step by step.
"1 plus 2 is?"
"3!"
"Then 5 plus 4?"
"9!"
"Try subtraction? 7 minus 3 is?"
"4!"
"...3 times 4?"
"12!"
"Division? 20 divided by 5?"
"4!"
My pen stopped. After a moment's thought, she wrote an equation on the paper.
[x + 3 = 7]
Nah, that's too much. Don't push it—stick to basics...
As I thought that, A-yeon answered right away.
"x is 4!"
No way, so I wrote another.
[2x - 1 = 9]
"x is 5!"
"..."
My hands started shaking. I wrote fractions. Negative numbers. Squares. Square roots. Even derivatives and integrals.
She nailed every single one. Instantly.
"...A-yeon."
My voice cracked without me realizing.
"Where did you learn this?"