On May 26, 2025, a scholarly paper co-authored by Mr. Soma Hayashi and Associate Professor Kaiji Motegi has been published from the Communications in Statistics – Theory and Methods. CSTM is a refereed international journal on statistics, and it is indexed in Science Citation Index Expanded with the 2023 Impact Factor being 0.6. Mr. Hayashi, currently an undergraduate junior of the Department of Economics, Kobe University, began to write this paper jointly with Associate Professor Motegi when he was a freshman.

In their paper, Mr. Hayashi and Associate Professor Motegi have established a new theorem on the birthday paradox, a classical problem in probability and statistics. Their theorem characterizes how conditional probability evolves in reaction to the arrival of new information, which has potential implications on security informatics and other fields. The new theorem is a novel generalization of the existing theorem of Motegi and Woo (2024, CSTM); see our previous news article. It is a remarkable achievement for an undergraduate student to publish his/her paper at an SCIE journal.

Bibliographic details:
Kaiji Motegi and Soma Hayashi (2025). A groupwise approach to the birthday paradox.
Communications in Statistics – Theory and Methods.
DOI: 10.1080/03610926.2025.2505586