In Huizhou, December brought the first snow of the year without any warning.
Chen Zhuo walked toward his dorm from the Science Department building, his hands stuffed in his pockets.
He had just left a conference room in the School of Mathematics.
That afternoon, he and Wu Tao had spent four straight hours at the blackboard working out a domain of convergence. Although they had finally managed to bridge the gap in their proof, the intense mental exertion left his temples throbbing.
With final exams approaching, most students at this hour were either grabbing seats in the library or burying their noses in textbooks in a study hall.
Just as Chen Zhuo reached the second-floor landing, he saw Wang Dayong leaning against the doorframe of Room 216.
He was holding a roasted sweet potato, freshly bought from an oven just off campus. He was peeling the skin while peering intently through the crack in the door, a fond, knowing smile playing on his lips.
"Dayong, what are you doing?"
Chen Zhuo walked over and called out quietly.
Wang Dayong turned his head. Seeing it was Chen Zhuo, he quickly swallowed the sweet potato in his mouth and said in a hushed voice,
"Xiaozhuo, you have to see this. Lu Jia is in here cooking up something big."
Chen Zhuo peered through the crack in the door.
With his back to the door, Lu Jia was standing in front of a whiteboard he’d dragged in from who-knows-where.
He held a black dry-erase marker, sketching a diagram on the board with the meticulous air of a serious academic.
On the left side of the whiteboard was a simple yet precisely scaled coordinate plane.
Labeled on it were the old library, the Physics Building, and the Third Cafeteria.
On the right side was a dense cluster of mathematical formulas.
Chen Zhuo pushed the door open, and Dayong followed him inside.
Lu Jia was so engrossed he didn’t even notice the two others behind him.
He was tapping several coordinates on the board with the tip of his marker, muttering to himself.
Chu Ge sat in a swivel chair by the window, a lollipop habitually dangling from his mouth.
With his arms crossed over his chest and his long legs crossed at the ankles on the edge of his computer desk, he was watching Lu Jia as if he were looking at an alien.
"Forget about my hash algorithm for a second."
Lu Jia pushed up the glasses that had slipped down his nose and turned to Chu Ge, his expression as solemn as if he were presenting a thesis proposal.
"What we need to solve right now is an encounter problem in the real-world dimension."
Chu Ge rolled his eyes and shifted the lollipop to the other side of his mouth, the stick tilting up at the corner of his lips.
Chu Ge spread his hands.
Lu Jia turned back around and drew a line with an arrowhead between the old library and the Third Cafeteria.
"After two straight weeks of covert observation, I’ve determined that the senior’s daily schedule is extremely regular."
Lu Jia wrote down a time interval next to the line.
"Every Monday, Wednesday, and Friday afternoon, she is guaranteed to be studying in the foreign language reading room on the third floor of the north library. Her departure times are concentrated between 4:45 and 5:10 PM."
Beneath that time interval, Lu Jia drew a standard bell curve.
"This is a very typical normal distribution."
Lu Jia tapped the peak of the bell curve with his marker.
"4:57 PM is the peak with the highest probability density. In other words, if I show up in the first-floor lobby of the library at exactly 4:57 PM every day, there is a 68.2 percent probability that I’ll run into her coming downstairs."
"But that’s not enough."
Lu Jia shook his head and wrote a new formula on the board.
"A chance encounter in the library only allows for a nod and a smile. It lacks a reasonable opportunity to strike up a conversation. The best-case scenario is waiting in line at the cafeteria."
Lu Jia took a deep breath, his eyes radiating an absolute faith in his mathematical tools.
"I’ve introduced a Markov chain model to calculate the transition probability of her dining preferences."
Lu Jia drew three state nodes on the board, labeling them A, B, and C.
"State A is the clay pot congee on the second floor, State B is the rice with toppings on the first floor, and State C is the malatang on the third floor. Given that she was in State B this Monday and transitioned to State A on Tuesday, based on the sample data I’ve collected over the past half-month, the probability of transitioning from State A to State C is 0.4."
Lu Jia quickly jotted down decimals between the nodes.
"The temperature dropped sharply today, and it’s snowing in Huizhou. Based on the negative correlation between temperature and caloric intake demand, I’ve increased the weight of State C by 0.15. Therefore, the composite probability of her going to the third floor of the Third Cafeteria for malatang today is as high as seventy-two percent."
Lu Jia turned around, his eyes shining as he looked at Chu Ge and, standing by the door, Wang Dayong and Chen Zhuo.
He didn’t seem to mind the two extra people; or rather, he was desperate for an audience to validate his theory.
"The final step is the walking speed equation."
Lu Jia wrote a velocity formula at the very bottom of the whiteboard.
"The senior’s average walking speed is 0.9 meters per second. If she leaves the library at 4:57 PM and heads to the third floor of the Third Cafeteria, I just need to leave my class in the Second Teaching Building at 4:59 PM and merge onto the main path at a constant speed of 1.2 meters per second. I will end up right behind her on the stairway landing between the second and third floors of the cafeteria."
Lu Jia capped his dry-erase marker and let out a long breath.
"The margin of error is less than two seconds. At that moment, all I have to do is say, in an extremely natural tone, ’What a coincidence, you’re here for malatang too?’
This opening line is absolutely flawless."
The dorm room was silent for a full ten seconds.
Wang Dayong took a bite of his roasted sweet potato, swallowed, glanced at the formulas on the whiteboard, then back at Lu Jia, a look of pure bewilderment on his face.
As for Chu Ge, he let out a very short laugh.
He took his legs off the desk, slid his swivel chair forward, and leaned in toward the whiteboard.

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