  In undergraduate classes, the potential flow that goes around a circular
cylinder is designed for complemental understanding of mathematical technique
to handle the Laplace equation with Neumann boundary conditions and the
physical concept of the multipolar expansion. The simplicity of the standard
problem is suited for the introductory level, however, it has a drawback. The
discussion of higher order multipoles is often missed because the exact
analytic solution contains only the dipole term. In this article, we present a
modified problem of the potential flow around a rectangle as an advanced
problem. Although the exact solution of this case is intractable, the
approximate solution can be obtained by the discretization and the optimization
using multiple linear regression. The suggested problem is expected to deepen
the students' insight on the concept of multipoles and also provides an
opportunity to discuss the formalism of the regression analysis, which in many
physics curricula is lacking even though it has a significant importance in
experimental physics.
