Daniel S. Freed in Quanta:
The mathematics community lost a titan with the passing last month of Isadore “Is” Singer. Born in Detroit in 1924, Is was a visionary, transcending divisions between fields of mathematics as well as those between mathematics and quantum physics. He pursued deep questions and inspired others in his original research, wide-ranging lectures, mentoring of young researchers and advocacy in the public sphere.
Mathematical discussions with Is were freewheeling. He wanted to get to the essence of the matter at hand, to understand and present new ideas in his own way. He constantly asked questions and provoked others to look deeper and more broadly. He knew no boundaries, which led him to forge deep connections between fields. Above all, he valued his freedom — the freedom to explore, the freedom to err, the freedom to create. A social mathematician by nature, he sought out and nurtured friendships with like-spirited collaborators.
More here.