Chen Zhuo walked under the shade of the trees, a water jug filled with cool water in his hand.
He pushed open the heavy doors of the old library.
Behind the circulation desk in the first-floor lobby, a librarian wearing reading glasses was dozing in a rattan chair. A nearby radio played Huangmei opera at a very low volume.
Chen Zhuo lightened his steps and went up the stairs to the third floor.
At the end of the hallway was the foreign periodicals reading room.
Chen Zhuo pushed the door open and went inside. A few ceiling fans whirred lazily on the ceiling.
The vast reading room was empty, without a single soul in sight.
No.
There was someone.
From the end of a row of bookshelves came the sound of metal casters rolling across the floor.
Chen Zhuo followed the sound.
A girl with short hair was bent over, unloading heavy journals from a pushcart full of old foreign books and slotting them onto the shelves by category.
Su Wei.
Chen Zhuo didn’t make a sound to disturb her. He just stood at the other end of the bookshelf, quietly watching her work.
Su Wei’s movements were remarkably efficient, possessing an almost mechanical precision.
She would pick up a book, glance at the call number on the cover, and then, without the slightest pause, turn and flawlessly slide it into its designated space.
Swift and precise.
The pile of books on the cart gradually dwindled.
Su Wei straightened up, let out a long breath, and wiped the sweat from her forehead with the back of her hand. She then turned and walked toward a large desk by the reading room window.
That was her home base.
On the corner of the desk sat a plastic water jug of astonishing capacity, a tall stack of scratch paper, and a copy of *Probability Theory and Mathematical Statistics* as thick as a brick.
The book’s pages were curled from use, densely covered in annotations made with red and blue ballpoint pens.
She sat down, twisted open the jug, and chugged a large mouthful of water. Then she immediately picked up her pen and buried her head in the long string of complex formulas on her scratch paper.
Chen Zhuo took a few steps closer, stopping two tables away from her.
"Fancy seeing you here."
The tip of Su Wei’s pen froze, and she looked up.
When she saw it was Chen Zhuo, her expression remained placid. She simply raised an eyebrow slightly and put down her ballpoint pen.
"Here to borrow a book?"
Her voice was clear, with a slightly raspy edge.
"Yeah."
Chen Zhuo nodded and walked up to her desk.
"I’m looking for some reviews on discrete topology and graph theory from the sixties and seventies. I don’t need specific papers, just the bound volumes of a few major math journals from that era."
Su Wei didn’t say a word.
She didn’t even turn to look at the sluggish 586 computer behind the circulation desk. With an old relic like that, just waiting for the cursor to start blinking after running a search took two minutes.
She just sat there and closed her eyes for a moment.
Less than three seconds later.
"Zone C."
Su Wei opened her eyes, her tone as calm as if she were recalling the location of an item in her own home.
"Third row of shelves, sixth cabinet from the left, bottom shelf. It should be in the third or fourth stack from the right."
"Also,"
Su Wei added.
"Those volumes are right across from a west-facing window. The afternoon sun is harsh, so their green covers have probably faded to white or lost their color by now. When you’re looking, don’t just go by the color. Check the call number. The prefix is O11."
Chen Zhuo looked at her and suddenly chuckled.
"You even factored in the sun-fading?"
Su Wei picked up her ballpoint pen again and looked down at her scratch paper.
"It’s just common sense. Any dark-covered book in that section will start to discolor after about three years. Go get them while it’s still bright out. The lighting on the bottom shelf is poor."
Chen Zhuo said no more and turned toward Zone C.
Following the coordinates Su Wei had given him, he crouched down and began searching through the pile of old books on the bottom shelf.
Sure enough, in the third stack from the right, he pulled out several heavy journals with covers that had been bleached white by the sun and whose edges were frayed.
Chen Zhuo carried the books back to the desk by the window.
He didn’t sit opposite Su Wei, but rather two seats away, picking a spot with good light.
He took a few blank sheets of A4 paper and a pen from his bag, laying them out on the desk.
He opened one of the Russian journals, which gave off a musty smell.
Pages full of Cyrillic letters and complex mathematical symbols intertwined, like a long-abandoned labyrinth.
Chen Zhuo’s eyes scanned rapidly across the pages, his mind automatically filtering and translating the content.
He was observing and learning how the mathematicians of the past constructed their logic.
The tip of his pen landed on the paper, making a faint rustling sound.
The reading room fell silent again.
Only the ceiling fans above continued to turn, and the occasional clear chirp of a bird drifted in from far outside the window.
The two of them occupied opposite ends of the long table, neither speaking another word.
They were like two fish hunting in the same patch of sea—sharing a space, but never interfering with one another.
Time trickled by.
The shadows of the sun stretched slowly across the floor.
Chen Zhuo stopped writing and stretched his stiff neck.
He was trying to redefine these ancient geometric propositions from an algebraic perspective.
He picked up his water jug and took a sip, his gaze sweeping to the other end of the long table.
Su Wei was holding an almost rigid posture, staring intently at the scratch paper in front of her.
Her brows were knitted tightly together, her pen spinning rapidly between her fingers. Occasionally, she would irritably cross out a long line of formulas on the paper, only to write an even longer one in its place.
Chen Zhuo set his water jug down, stood up, and walked over.
He didn’t deliberately soften his steps, stopping at the seat diagonally across from Su Wei.
Su Wei sensed the movement and looked up, her eyes still holding the frustration of an unsolved problem and a hint of annoyance at being interrupted.
"You look like you’re just grinding away there."
Chen Zhuo said it in a gentle tone, making a casual joke.
Su Wei ignored his lame joke, just pushing the thick copy of *Probability Theory and Mathematical Statistics* forward and saying, somewhat petulantly,
"This is what the book says. For risk assessment in financial markets, the time variable is inherently continuous. How can you get accurate data without using calculus to find the expected value?"
Chen Zhuo glanced at the dense thicket of integral signs and limit operations on her scratch paper.
It was a classic stochastic process problem used to evaluate the future trends of a certain financial derivative.
Su Wei was clearly trying to solve it by strictly following the continuous-time Brownian motion model from the textbook, but the sheer complexity of the calculations had obviously exceeded the limits of manual computation, leading her into a dead end.
"The desire to make big money is strong, but your choice of tools is a bit clumsy."
Chen Zhuo pulled out a chair and sat down opposite her, as if it were the most natural thing in the world.
"What do you mean?"
Su Wei watched him warily.
Chen Zhuo didn’t explain. He simply reached out and pointed to the lengthy integral formula on her scratch paper.
"Your premise is wrong. In a real financial market, trading figures do change over time, but that doesn’t mean you have to treat it as a perfectly smooth curve."
Chen Zhuo picked up a spare pencil from the desk and pulled the paper in front of him.
"If you insist on following this continuous line, you’ll be drowned in endless minute fluctuations. Calculating this by hand? That’s a job for a computer. I’m sure you could manage, but it’s a waste to use your brain on something like this."
In a blank space on the paper, he deftly drew several circles and connected them with straight, arrowed lines.
"Change your perspective."
Chen Zhuo’s voice was steady and clear, as if he were stating a simple piece of common knowledge.
"Don’t look at the line. Look at the points."
"Break up the continuous timeline. Assume the market only exists in a few key states: it goes up today, down tomorrow, or stays flat. Each state transition only depends on the one immediately preceding it, and nothing further back."
Chen Zhuo wrote a few simple probability values next to the circles, then enclosed them in square brackets to form a square matrix.
"Discretize it. Turn it into a transition probability matrix. Then, all you need to do is calculate the n-th power of this matrix."
He put the pencil down and pushed the scratch paper back to Su Wei.
"A Markov chain?"
Su Wei’s eyes lit up slightly as she looked at the exceptionally clean square matrix on the paper.
"Correct."
Chen Zhuo leaned back in his chair.
"You reduce a complex continuous problem into a discrete algebraic one. The computational load is at least halved, and in terms of error tolerance, it’s far superior to your calculus, which is precise to several decimal places."
Su Wei didn’t speak. She picked up her pen and, following the matrix Chen Zhuo had drawn, tried to plug in the data from her problem.
A few minutes later.
The computational bottleneck that had her completely stuck was sliced open as if by a sharp dagger.
The complex integration process was perfectly replaced by matrix multiplication, and the final expected value appeared clearly on the paper.
Su Wei put down her pen, looked at the result, and let out a long breath.
She looked up, her gaze on the Chen Zhuo sitting opposite her now complicated.
"How are you so familiar with the underlying logic of financial actuarial science?"
Chen Zhuo smiled.
"I don’t know anything about finance, but I know math. And I know how to be lazy when circumstances permit."
He pointed at the bright sun outside.
"On a hot day like this, brain cells are a non-renewable resource. If you insist on using the most cumbersome method to calculate, you’ll get heatstroke. In the end, those formulas are just tools. If a hammer doesn’t work, why not try a wrench?"
Su Wei looked at him, and for the first time, the corner of her mouth twitched into a very faint smile.
"Thanks."
It was a stiff thanks, but a sincere one.
"You’re welcome."
Chen Zhuo stood up and pointed to the distant bookshelves.
"Since we’re even now, could you help me with one more thing? The *American Mathematical Society Bulletin*, the 1975 bound volume. Do you have it?"
Su Wei didn’t even hesitate.
"Zone F, first row, top shelf. You might need a small stool. Those volumes are very thick and placed up high."
"Okay."
Chen Zhuo turned and headed for Zone F.
Su Wei watched his retreating back, then looked down again at the concise and powerful matrix on her scratch paper.
’This is a more efficient, more ruthless path.’
Chen Zhuo returned with the thick *American Mathematical Society Bulletin* and sat back down in his spot.
Outside, the sky gradually darkened. The halo of the setting sun shone through the windowpanes, casting long, orange-red bands of light on the floor.
The reading room once again returned to the sounds of whirring fans and rustling paper.
Chen Zhuo wrote down the last line of the matrix’s eigenvalues on his scratch paper.
He put down his pen, neatly folded the few pages filled with formulas, and tucked them into his notebook.
After packing his things, Chen Zhuo picked up his water jug and prepared to leave.
As he passed by Su Wei’s desk, she was still hunched over, frantically performing matrix calculations.
"I’m heading out."
Chen Zhuo said casually.
"Mm."
Su Wei didn’t look up.
"Are you coming back tomorrow?"
"Yeah, I still have a lot to read."
"Okay. If you need to find anything tomorrow, write down the year and general category on a slip of paper for me beforehand."
Su Wei’s tone was calm, as if she were coordinating a work task.
"Okay."
Chen Zhuo pushed open the wooden door of the reading room and stepped out into the summer evening breeze.

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