Article date: Sep 1965
Article title: The "superellipse": a curve that lies between the ellipse and the rectangle
Mathematical Games column? Y
Book: 7. Mathematical Carnival
Chapter: 18. Piet Hein’s Superellipse
This data confirmed? yes
Notes
Peter Rowlett (27 August 2022):
Peter Rowlett initial guess (Uncertain)
Peter Rowlett (29 October 2023):
I have seen this chapter and column side-by-side and they match.