[No. 125] Takeshi Kurosawa

I had always been relatively good at math and considered it a favorite subject, but a small-group class called Mathematics Research Methods, offered to second-year students, made me realize this more deeply. In this class, the instructor would assign a research topic, giving us undergraduates a chance to experience mathematical research (Reference: *Mathematics Research Methods* by Hitoshige Kuroyama).

My instructor for the class was Professor Koyama (now a professor at Toyo University), who specialized in number theory (particularly ζ-functions). Number theory has many problems that are easy to get into (though difficult to solve), and I tackled a simple question: while the even values of the ζ-function, ζ(2m) (m∈ℕ), have an explicit expression as a power of pi (π) multiplied by a rational number, can ζ(3) also be expressed explicitly? To this day, the only fact that has been shown is that ζ(3) is an irrational number, and the values for odd integers remain a great mystery.

This was not a problem an undergraduate could be expected to solve, but youth has its advantages. Without a second thought, I went to the library to borrow master's theses and other materials, devouring various documents and getting a taste of research, albeit a naive one. I went through cycles of thinking I had a breakthrough, only to later find a mistake and be disappointed. In the end, I couldn't solve anything, but my research report, through Professor Taka-aki Tanaka (now an associate professor at Keio University), led me to my greatest mentor, Professor Iekata Shiokawa (now Professor Emeritus at Keio University). Professor Shiokawa, whose research focused on irrational and transcendental numbers, specially allowed me to join his seminar from my third year, and that is how my mathematical research began.

When I advanced to the master's program, a brilliant student from Tohoku University named Yohei Tachiya (now an associate professor at Hirosaki University) joined the Shiokawa lab as a new member of my cohort. It would be presumptuous to call him a rival, but he was more of a brilliant type than just a diligent one, and I learned a great deal from him. He lived in a boarding house near the university, and since I often found it a hassle to go home, I frequently imposed on him. His studio apartment had a huge whiteboard, and the two of us often studied for our seminars there. That's not to say we studied around the clock; in fact, we probably spent most of our time playing two-player mahjong, train games, or online mahjong. We would stay over at his place, then he would stay at my family's home, and we once spent more than a week eating and sleeping under the same roof. He was the friend I spent the most time with outside of my family. I have fond memories of those student days, like when we would stay up all night playing online in the lab, and the cleaning staff would say "Good work" to us. We'd feel a bit embarrassed but reply, "We got a lot done today," and share a wry smile.

Career paths after completing math Doctoral Programs were relatively limited, and a university post was not guaranteed. I felt that Professor Shiokawa was encouraging me to continue my studies, and although I had a strong desire to advance to the doctoral program, I was anxious about the future and chose to take a research position at a company.

I was assigned to one of the Research Centers and Institutes, so the content of my work changed completely, and I found myself doing research that resembled statistical analysis. However, a few years after graduating, I couldn't forget my desire to properly master the mathematics I had worked so hard on. I consulted Professor Shiokawa, who readily accepted me into the doctoral program, and I began a double life as a working professional and a doctoral student. On weekdays, I would handle my company duties (research), then switch gears to think about math on my commute, scribbling down notes for proofs. On Saturdays, I had seminars with Professor Shiokawa, and on Sundays, I would write up my work. I lived this way for two and a half years. It was certainly a demanding time, but I've never been good at sitting still to study and have always enjoyed studying on the train, so it wasn't a hardship in itself. With the cooperation and understanding of my wife (as I was already married by then), and with Professor Shiokawa taking the time to mentor me every Saturday, I managed. My child was also born while I was in the doctoral program, which was a hectic time with moving and other things. At my dissertation hearing, I discovered a mistake in my submitted documents for the Doctoral Programs. I was told, "You must be tired from being so busy," which I found very embarrassing, but I somehow managed to complete the program.

After obtaining my degree through various struggles, I applied for open positions and was fortunate enough to find a university post, which led me to my current job. Although I took a relatively unusual path to an academic position, I am reminded daily that I am who I am today thanks to the immense help of many people, including those not mentioned here. I would like to take this opportunity to express my gratitude.