By Gene Allgower, Annette Boehmer, Mei Zhen, and Bernhard A. Schmitt
Klaus Boehmer, 1936-2020.
Klaus Boehmer, emeritus professor of mathematics at the Philipps University of Marburg in Germany, passed away on March 25, 2020, in the loving care of his second wife after battling severe illnesses. He was nearly 84 years old. Klaus was a prolific, gregarious mathematician whose career produced numerous publications, books, conferences, and lectures around the world. He left behind his second wife, two daughters, and five grandchildren, and took dedicated, loving care of his first wife before she passed away.
Klaus was born in Karlsruhe, Germany, on April 20, 1936. His mother was an infant nurse, and his father was procurist of a company called Ebersberger & Rees and later general manager of a protestant organization called Das Diakonische Werk. Klaus was the oldest of four boys.
When Klaus started secondary school, his mother received a call from the headteacher, who told her that she should take Klaus out of school as soon as possible as he was completely useless, especially in maths.
In 1956, Klaus proved that teacher wrong by studying maths, physics, and philosophy at the Technische Hochschule Karlsruhe and Hamburg University. He finished his studies in 1962 with a Diplom in mathematics, then earned his Ph.D. in 1969 and Habilitation in 1972, both in Karlsruhe.
After receiving offers of C3 professorships from Paderborn and Göttingen, Klaus became a C3 professor in Karlsruhe from 1974 to 1980. In 1980, he took a job as professor of numerics in a C4 position at the Philipps-Universitaet Marburg; he remained there until his retirement in 2001.
Throughout his career, Klaus was dedicated to encouraging his students — he even directed seven of them to their Ph.D.s.
Upon finishing his Habilitation at the Karlsruhe Institute of Technology and becoming a professor in the affiliated Institute for Applied Mathematics, Klaus authored a textbook on spline functions. He then began working on what would become the principle thread of his career: the numerical solution of nonlinear differential equations, partial differential equations, and bifurcation problems. Using asymptotic expansions with the help of extrapolation methods, Klaus analysed the defect correction methods in conjunction with Hans Stetter. He became interested in the interplay of discretizations of differential equations and Newton’s method for solving systems of equations that arise from the discretizations. This led to an analysis of what became known as discrete Newton methods. In turn, Klaus partook in investigations with Gene Allgower and Stephen McCormick for refinements of finite difference methods and the application of Newton’s method, thus resulting in the so-called mesh independence principle.
Klaus was able to deftly analyze the convergence of many types of discretizations of nonlinear systems for differential and partial differential equations. Up to the end, he was investigating center manifold theory for bifurcation problems and using radial basis discretizations with Robert Schaback.
A culmination of his scientific life in several senses was certainly his research-level monograph entitled Numerical Methods for Nonlinear Elliptic Differential Equations, which was published by Oxford University Press in 2010. Among other things, it demonstrated his ability to cooperate with many people. The monograph has four coauthors, and Klaus was in contact with many others while preparing the second volume, which now unfortunately will not appear. The publication also displays Klaus’ broad scope and readiness to approach new problems, since he and his coauthors cover difference methods, conforming and non-conforming finite element methods, discontinuous Galerkin methods, and wavelets; these topics are all based on a general discretization theory that Klaus developed over the years.
Klaus’ wide-ranging mathematical interests led to contacts and invitations around the world. Besides several stays in the U.S., he visited China multiple times to lecture and travel. He loved the people, the food, and the amazing nature. In 1984, Klaus was one of the first foreign professors to ride a bicycle among thousands of Chinese citizens during rush hour to present a lecture at Xi’an Jiaotong University.
Klaus invited many colleagues to participate in joint research projects, and often hosted them at his home to reduce costs for his university. He was also a dynamic lecturer and an enthusiastic organizer of conferences, proceedings, etc.
Klaus’ passions extended well beyond mathematics. He was involved in his church, served on its construction committee, and was actively engaged with the procurement of the new organ. He held a regular music quartet at his house and brought his violin to conferences to organize musical evenings. Klaus was known around Marburg as the professor who rode his bicycle through the forest and hills between his house and the lecture halls at impressive speeds. He was very sporty and fit throughout his life. Klaus was always optimistic; loved a good laugh, good food, cheese, and wine with family and friends; and possessed the never-ending energy to tackle new projects.
Klaus was very talented and an all-around fine mensch. We will miss him dearly.
Gene Allgower is a professor emeritus at Colorado State University. His principle area of study is numerical continuation methods and the numerical solution of nonlinear systems. Annette Boehmer of Marburg worked for Schlumberger for 24 years until her recent retirement. Mei Zhen is the president of Manifold Data Mining Inc. and a former Ph.D. student of Klaus Boehmer. Bernhard A. Schmitt is a retired associate professor at the University of Marburg. His research is focused mainly on numerical methods for stiff ordinary differential equations.