Joint Colloquia

ASIAA/CCMS/IAMS/LeCosPA/NTU-Phys/NTNU-Phys

Coordinators:

Chia-Hung Chang (NTNU Phys), Ssu-Yen Huang (NTU Phys), Shao-Yu Chen (CCMS), Cheng-Tien Chiang (IAMS), Jiwoo Nam (LeCosPA), Tomomi Sunayama (ASIAA), Shang-Min Tsai (ASIAA)

Sep. 8, 2026 (week 01)

Wui-Tet Yiunn

Professor Emeritus, Department of Mathematics, National Taiwan University

Tidal Force and its Eigen-Problem

Host: Proty Wu

Time: 2:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

A uniform ball-body B generates its Newtonian gravitational field g. When the average gA of this field over another separate ball-body A is subtracted from g, the difference is called the tidal force t := g − gA , on A due to B. A point P on the spherical surface S of the ball of A is called an eigenpoint for t, if t(P) is vertical to S, so that t(P) is the unit vector along the radius vector OP multiplied by a scalar λ, the tidal eigen-value. We present a solution of this eigenproblem via a natural Lamé ’s diagram △TQO for the trio (g(P),−g(O), t(P)). The solution is radically so elementary that a kid just entering senior high school can follow it. There arises the natural problem to find a simple oriented path Γ on the surface S, along which the integral of t is a maximum. We reduce this problem to a constrained optimization problem for the tidal-potential Φ of t, and the Lagrangian prescription for it is identical to tidal eigenproblem.

Brief Bio

Professor Wui-Tet Yiunn (楊維哲) received his B.Sc. from National Taiwan University (NTU) in 1964, and his Ph.D. from the Department of Mathematics at Princeton University in 1971. Then he joined the Dept. Math. NTU as an associate professor. He was the department chairman till 1973, and promoted to a professor in 1974. He served as President of the Taiwan Association of University Professors (TAUP) in 2001. But beyond this little involvement in public affairs and a general academic interest in mathematics, Professor Yiunn has great enthusiasm for the preservation of Taiwanese culture. He co-founded the SiTien Foundation for Taiwanese Puppet Show (西田社布袋戲基金會) alongside Professor Jin-Tze Chen (陳金次) and Professor Hong-Hi Li (李鴻禧) in 1985, chaired the board of trustees for a little longer than a decade. Also notably, he is known for teaching the course of Mathematical Methods of Reasoning, and the Calculus course, in Taiwanese (台語), in 1997-1999.