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Ronald Jensen (1936 - 2025)

Ronald Jensen (April 01, 1936-September 16, 2025)

Ronald Jensen was born in Virginia in the US in 1936. After having completed a BA in economics in New York in 1959, he decided to move to Europe and become a logician. He finished his Ph.D. with G. Hasenjaeger at the University of Bonn in 1964 with a thesis on subsystems of arithmetic, and he did his Habilitation at Bonn in 1967 on levels of the constructible hierarchy.

Jensen developed the fine structure theory all by himself, isolated diamond, square, and other combinatorial principles which he showed to hold in L, and he used his machinery to present his Covering Lemma for L as "Marginalia to a Theorem of Silver." He designed various forcing techniques, one of the most sophisticated ones being the coding into a real apparatus. In joint work with T. Dodd he built the first core model, an enterprise which was later pursued by W. Mitchell, J. Steel, and others. He later worked on the theory of larger core models. He also isolated the notions of "subproper" and "subcomplete" forcings and variants thereof, and he produced his own iteration theory for them. He designed "L-forcing." In his last years, he worked on a book on the core model below one Woodin cardinal.

Jensen held positions at the Rockefeller University, at UC Berkeley, at the University of Bonn, the University of Oslo, the University of Freiburg, the University of Oxford, and the Humboldt University of Berlin, from which he retired in 2001.

Jensen was the first Gdel lecturer of the Association for Symbolic Logic in 1990. In 2001, he gave the Tarski lectures at UC Berkeley. In 2003, he reveived the Steele price. In 2015, the European Set Theory Society awarded him and J. Steel the Hausdorff Medal for their joint paper "K without the measurable."

On September 16, 2025 Ronald Jensen passed away. He left us his life's work mostly in the form of thousands of pages of handwritten notes. A lot of current-day set theory consists in generalizing concepts and arguments which go back to Jensen. Set theory lost one of its giants.

(written by Ralf Schindler)
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