The mathematics behind Inge Lehmann’s discovery that the inner core of the Earth is solid is explained using data collected around the Earth on seismic waves and their travel time through the Earth.
Christiane Rousseau (rousseac@DMS.UMontreal.CA) is a professor at the University of Montreal, past President of the Canadian Mathematical Society (2002–2004), and currently Vice-President of the International Mathematical Union (until 2014). Her research is in dynamical systems. A passionate popularizer of mathematics, she is the initiator and coordinator of the special year Mathematics of Planet Earth 2013. Her interest in pre-service teacher education led her to write the book Mathematics and Technology, jointly with Yvan Saint-Aubin. She writes, “One of my pleasures when working for MPE2013 is that I learn new applications on a regular basis. I knew that Inge Lehmann discovered that the inner core of the Earth was solid, and I wanted to understand the idea, so I read her paper. It is not necessarily easy to read a paper in another discipline, here geophysics. Fortunately, near the end, Lehmann sketches a simple model of the Earth, which she used to sell her discovery. I was stimulated to fill in the mathematical details and missing calculations. I found them sufficiently interesting to share.”