Ducky was a shadow.
When Seoha opened his eyes, he vanished.
“Stay quiet. Don’t come out.”
Whenever he whispered that, Ducky had to hide himself in the darkness.
But Ducky did not disappear. The more he was suppressed, the sharper and more tenacious he became. He kept knocking at the door.
Ducky was the inner voice he had tried so hard to ignore.
Ducky was compulsion.
The shackles that threatened the things he loved, the monster he had always feared.
But today, Seoha sought him out first.
“Ducky, let’s begin.”
At those words, Ducky walked out into the light.
Not as an impulse that had to be restrained, but as an invited companion.
Ducky was the tenacity Seoha had lost, and the eye that analyzed the world.
Ducky emerged from the deepest part of Seoha’s consciousness and became one with him. And his scattered thoughts joined together like a vast river.
Seoha trembled.
The thrill of liberation that came from being allowed to reveal his entire self.
When he opened his eyes, the world had changed.
The hazy fog lifted.
And everything appeared clearer than before.
He picked up Calculation Theory, the book he had borrowed from the library. Numbers and symbols writhed before his eyes as if they had come alive.
And his thoughts accelerated.
But clarity was also discomfort.
His mind spun without rest like an overheated machine, trying to analyze everything in sight. His breathing grew rapid. His nerves were pulled taut, as if the slightest additional strain would snap them.
But just for today, Seoha decided not to care.
Seoha walked to the blackboard and picked up a piece of chalk. Then he began writing equations in one breath.
χ(G)=min{k∈N∣∃f:V→{1,···,k},f(u)≠f(v)∀(u,v)∈E}
∀ planar G,χ(G)≤4
“Every graph has a weakness.”
“A vertex of low degree. Five, six at most. Let’s break it down from there.”
Countless figures were built up and collapsed inside his head.
If he plucked out a single vertex, the remaining figure would find stability within four colors.
But in one place, where five branches were tangled together, a crack formed. Colors overlapped and lines twisted.
The hand holding the chalk did not stop as it drew new lines.
The patterns increased. Exceptions piled up.
The blackboard was soon covered in graphs and figures. Seoha brought over a whiteboard from the side, set it up, and continued thinking.
Seoha was wandering through a forest of graphs.
No matter where he went, he could not find an exit.
“Hello?”
A graph popped into view before his eyes. Then they overlapped one another and multiplied like monsters.
As if mocking him, asking whether he really thought he could face it with only that.
Countless branches, countless patterns, countless exceptions.
It was an immortal that would never die.
But all of this was within his expectations. Seoha did not stop his hand.
“Don’t make me laugh. You’re dying here today.”
Seoha’s lips twisted grotesquely.
As the night deepened, the exceptions stacked up neatly on one side of the blackboard. Seoha brought over another board and opened up a new space. Then he began rewriting the exceptions as a series of symbols and rules.
‘Encode only the information on the boundary.’
The exceptional figures were newly substituted.
As if reducing a vast forest into a single map, the infinitely expanding patterns were organized into a single automaton diagram.
The chalk scraped rapidly as it filled the blackboard.
[State Set S], [Initial State q₀], [Transition Function δ]···.
And at a certain moment, the automaton drawn on the blackboard closed its own circle and defined its range.
The forest of exceptions no longer spread infinitely.
“Thirty-two.”
Seoha murmured in a low voice.
The countless exceptions had ultimately been compressed into thirty-two patterns.
“It’s not over yet.”
This was only the bare minimum condition for solving the four color theorem.
* * *
Click.
When he pressed the button, Gyeoul’s performance flowed out of the CD player.
Seoha sat in a chair, his face haggard.
He looked as if he might collapse at any moment, like a runner who had just finished a marathon.
His hand shook as he picked up a water bottle.
He barely managed to hydrate himself and took out an energy bar.
“Huuuuuu.”
Seoha leaned his head back and let out a deep breath.
His trembling fingertips were stained with chalk dust, and his shoulders, stiff with tension, felt as heavy as stones.
But this was only the beginning.
Gyeoul’s performance gently soothed his nerves.
And slowly, his breathing began to settle.
Rattle.
When he drew up the blinds, the sun was rising. Seoha narrowed his eyes and turned his head away.
“Let’s move on to the next stage.”
The time he could use this place was limited. There was no room for rest.
Seoha rose from his seat.
The blackboard was packed with the figures and symbols he had built up overnight.
He looked around. There was not enough space to write.
After thinking for a moment, Seoha opened a box and took out A3 paper. Then he began taping the sheets to the wall one by one.
A short while later, a massive board had appeared on the wall.
Now it was time to verify the soundness of the rules he had made.
Seoha interlocked one of the compressed patterns with another. Then a flow of colors emerged, moving back and forth across the interior and the boundary. Through the equations, Seoha could see with his own eyes the four colors mingling with one another.
‘As if it would go this smoothly.’
The color that had been flowing along smoothly suddenly choked to a halt.
To Seoha, it felt as if the monster was threatening him, telling him not to come any closer.
‘Not a chance. I just need to strengthen the rules.’
One, then another.
Before he knew it, the wall was filled with the equations Seoha was drawing.
Seoha did not rest until every boundary fit together perfectly.
And so, two days passed.
* * *
On Monday morning, the warning bell for class rang.
The students in the classroom moved busily as they found their seats. But the student who was always there before anyone else was nowhere to be seen.
The child who always sat with his textbook open, staring straight at the blackboard; the student who had never once been late or absent.
Everyone could feel that absence.
“Seoha still isn’t here?”
“I don’t think I’ve seen him today.”
“That’s weird. Try calling him!”
The students’ murmuring spread.
Rattle.
The homeroom teacher opened the door, entered, and stood at the lectern.
His gaze swept once over Seoha’s seat.
“I was told Seoha called in sick today. Does anyone know anything?”
The students looked at one another with puzzled expressions.
“Did you hear anything?”
“No.”
“Seoha looked perfectly fine on Friday.”
A crease formed between the homeroom teacher’s brows.
What he had received yesterday was a single short text message.
-Teacher, I’m not feeling well, so I think I’ll have to be absent on Monday.
-What hurts? Have you been to the hospital?
Sick leave was not granted that easily.
Either the teacher had to personally confirm the student’s condition, or there had to be some kind of medical record.
But no reply came.
He did not answer when called, either.
He had a bad feeling.
“Taejin, Gibeom, go check Seoha’s dorm room. If he’s collapsed or anything, call me right away and let me know.”
““Yes!””
The two students sprang up and left the classroom.
The dormitory, emptied of students, was strangely quiet.
Bang, bang!
“Seoha! Open the door!”
Gibeom looked at Taejin. Taejin nodded.
“Seoha? We’re coming in, okay?”
Beep—
When they held up the spare key they had gotten from the dorm supervisor, the door opened.
“Huh?”
“He’s not here?”
Taejin looked around.
It was a room where not a single object was out of place, just like Seoha’s usual personality.
His laptop was placed neatly on the desk, and his textbooks and reference materials were all in their proper places.
A little while later, they returned to the teachers’ office and reported the situation.
“He’s not there?”
The homeroom teacher’s voice rose.
“Let’s check with the management center.”
The head of student affairs, who was standing nearby, took out his phone and dialed a number.
“Yes, please check the student entry and exit records. First year, Class One, Yu Seoha.”
After a brief silence, the voice of a staff member came through the receiver.
“What did you say? The advanced curriculum seminar room?”
The head of student affairs’ eyes widened.
It was very rare for a student to use that place on the weekend.
“I’ll go check.”
The homeroom teacher shot up from his chair.
“I’ll go with you.”
The head of student affairs immediately followed him at a brisk pace.
Yu Seoha was a student the school managed specially. He wanted to see with his own eyes what had happened.
* * *
“Oh, my aching bones.”
Professor Kim Jiyun tapped her shoulder with the hand that wasn’t on the steering wheel.
Seoha had said he was doing some research, so she had planned to visit him there over the weekend, but an urgent matter had come up.
Only now had things settled down, and she intended to stop by the school, even if late, before her afternoon lecture.
At that moment, her phone rang.
Trillllll—
Jiyun pressed the button on the steering wheel.
“This is Kim Jiyun.”
-Professor, I’m sorry, but could you come to the school immediately?
It was the principal’s urgent voice, without the slightest trace of composure.
In that instant, Seoha flashed through Jiyun’s mind.
“What happened? Please explain.”
-Yu Seoha is···.
“What about Seoha?”
-It’s difficult to explain in words. All of us teachers agreed that it would be best if you came first···. It’s the seminar room where you always teach.
“I was already on my way. I’ll be there in ten minutes.”
Jiyun stepped on the accelerator.
Vroooom—
Not long afterward, she arrived at the school.
After parking her car haphazardly in the lot, Jiyun hurried out and began to run.
‘What on earth happened···.’
She could only hope that Seoha was safe.
When she climbed the stairs, she saw a group of people blocking the hallway.
The principal, the vice principal, teachers, and even some students were mixed in.
When the principal saw her, he ran over in a panic.
“Professor, please take a look at this. What on earth is Seoha doing right now?”
Before even hearing her answer, he grabbed Jiyun by the sleeve and pulled her along.
The seminar room door was wide open.
When Jiyun stepped inside, she was so startled she almost screamed.
Paper had been plastered over three walls of the classroom.
Every sheet was densely filled with equations, symbols, small figures, and graphs. To her eyes, the equations were still in progress.
Arrows crisscrossed in every direction, while circles, triangles, and graphs tangled like chains collided across the walls and generated meaning.
On the enormous blackboard at the front, an automaton diagram had been completed.
Dozens of circles were connected by arrows, and each stage had colors or symbols attached to it. It felt as though it were pulsing on its own, like the heart of a giant monster.
Only now did Jiyun understand the principal.
Anyone would be frightened when faced with such an overwhelming sight. None of them understood the meaning of what Seoha was doing. That had to be why they had called her.
When Jiyun tried to enter the classroom, the principal raised his arm to stop her.
“You can’t! Seoha is extremely sensitive right now. We tried to go in several times too, but failed.”
Jiyun gave a faint laugh.
“I’ll be fine.”
‘No one here may understand you, but I do.’
The four color theorem.
As a Fields Medal candidate and the nation’s foremost cryptography expert, she could see through to the true meaning of the immense diagram Seoha had created.
“Don’t come in!”
It was a sharp voice, completely unlike Seoha.
Jiyun stopped where she was and looked around. Discarded sheets of paper were scattered everywhere.
“I won’t disturb you.”
At Jiyun’s gentle voice, Seoha briefly stopped the chalk that had been moving so busily.
“It’ll be over soon. It won’t take long, so please wait just a little.”
How many days had he been at this?
His pale face. His trembling fingertips.
Seoha looked as if he might collapse at any moment.
And yet his eyes alone shone so fiercely it sent chills down the spine, and no one dared approach him.