Dr. James Simons was President of Euclidean Capital, a family office, and Board Chair of Renaissance Technologies LLC, a highly quantitative investment firm from which he retired in 2009 after many years as CEO. Previously, Dr. Simons was chairman and professor of the Mathematics Department at the State University of New York at Stony Brook. Earlier in his career he was a cryptanalyst at the Institute of Defense Analyses in Princeton, and taught mathematics at the Massachusetts Institute of Technology and Harvard University. He died on Friday May 10 2024 in his home in Manhattan. He was 86. Dr. Simons held a B.S. in mathematics from the Massachusetts Institute of Technology and a Ph.D. in mathematics from the University of California at Berkeley. His scientific research was in the area of geometry and topology. He received the American Mathematical Society Veblen Prize in Geometry in 1975 for work that involved a recasting of the subject of area minimizing multi-dimensional surfaces. A consequence was the settling of two classical questions, the Bernstein Conjecture and the Plateau Problem. Dr. Simons' most influential research involved the discovery and application of certain geometric measurements, now called the Chern-Simons Invariants, which have wide use, particularly in theoretical physics. Dr. Simons is the founder and chairman of Math for America (MƒA). He serves as Trustee of Brookhaven National Laboratory, the Institute for Advanced Study, Rockefeller University and the Mathematical Sciences Research Institute in Berkeley. Dr. Simons is also a member of the Board of the MIT Corporation and Chair Emeritus of the Stony Brook Foundation. Together with his wife, Marilyn, Dr. Simons manages the Simons Foundation, a charitable organization primarily devoted to scientific research. The Foundation’s philanthropic activities include a major research initiative on the causes of autism and the recent establishment of an institute for research in mathematics and theoretical physics. His first marriage, to Barbara Bluestein, a computer scientist, with whom he had three children — Elizabeth, Nathaniel and Paul — ended in divorce. He then married Marilyn Hawrys, an economist and former Stony Brook undergraduate who received her doctorate there. They had two children, Nicholas and Audrey. Paul Simons, 34, was killed in a bicycle accident in 1996, and Nicholas Simons, 24, drowned off Bali, Indonesia, in 2003. His wife and other children survive him, as do five grandchildren and one great-grandson.