**Åke Pleijel** was a Swedish mathematician who worked on differential equations and potential theory.

- Pleijel graduated from the high school in Östermalm, Stockholm on 11 May 1932 and enrolled at Stockholm University on 23 May 32.
- After graduating with his first degree, Pleijel began undertaking research advised by Torsten Carleman.
- Pleijel extended his investigations on the asymptotic behaviour of eigenvalues and eigenfunctions for certain oscillation problems to a number of other areas, including strained plates.
- Let us record at this point that Agneta Christina Pleijel became an author, poet, playwright, journalist and literary critic; Sonja Carolina Pleijel studied at Lund University and became a TV producer, director and screenwriter.
- On 14 September 1940 Pleijel was appointed as a docent in mathematics and an assistant in the mathematics library at the University of Stockholm, a position he held until he was appointed as a docent at Lund University on 25 April 1941.
- While at Princeton, Pleijel worked with Subbaramiah Minakshisundaram (1913-1968), an Indian mathematician with an expertise in parabolic partial differential equations.
- In fact, the paper contains the famous zeta function, now called the Minakshisundaram-Pleijel zeta function, which is a zeta function encoding the eigenvalues of the Laplacian of a compact Riemannian manifold.
- Back in Sweden, Pleijel took up his appointment as professor of mathematics at the Royal Institute of Technology in Stockholm on 1 January 1949, where he succeeded Johannes Malmquist, who had retired in 1948.
- Pleijel had actually been appointed to this position on 22 October 1948 while he was at Princeton.
- The 1959 International Handbook of Universities lists Lund University with Åke Pleijel as Dean of the Faculty of Mathematics and Natural Sciences with a staff of 102.
- This decision was endorsed by the Swedish Mathematical Society on 30 November 1958 and a joint invitation was issued to the mathematicians of the world, signed by the chairmen of the National Committee and the Society, A Pleijel and G Borg.
- O Frostman and A Pleijel were entrusted with the initial preparations for the congress together with H Cramer, at that time Chancellor of the Universities of Sweden, and Lars Gårding, all of whom later constituted the Organising Committee.
- From 1965 to 1967 Pleijel went to Ethiopia as a representative of UNESCO.
- Pleijel authored several textbooks for Ethiopians and, perhaps most importantly, he managed to interest some young Ethiopians in mathematical research.
- On 30 June 1967, Pleijel was appointed as Professor of Mathematics at the University of Uppsala.
- At the University of Uppsala, Pleijel was dean of the Faculty of Mathematical and Natural Sciences from the autumn term of 1971 to the spring term of 1973.
- In this area, a collaboration developed with the University of Edinburgh, in which several of Pleijel's co-workers in Uppsala participated.
- One sign of great appreciation from the Scottish partner was the election of Pleijel as member of the Royal Society of Edinburgh.
- Pleijel published around 50 papers on topics such as: Differential equations, operational calculus; Ordinary differential equations; Partial differential equations, and Potential theory.
- Several of the mathematical concepts that Pleijel introduced are now known by his name and have been the subject of many papers.
- These include, in addition to the Minakshisundaram-Pleijel zeta function we mentioned above, the "Pleijel's nodal domain theorem", the "Pleijel's constant", "Pleijel's estimate", the "Pleijel inequality", and the "Pleijel identity".

Born 10 August 1913, Johannes parish, Stockholm, Sweden. Died 24 September 1989, Västerled parish, Stockholm, Sweden.

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Origin Sweden

**O’Connor, John J; Robertson, Edmund F**: MacTutor History of Mathematics Archive