Han's father spent the 1935--36 academic year on sabbatical in the United States. During that trip the Sah family became friends with Prof. William Everitt and his family. When Han's father died in 1949 Han and his brother Chih-Tang went to live with the Everitts in Urbana, Illinois, where William was Dean of Engineering.
In Urbana, Han came into the university's experimental high school as a sophomore knowing very little English, but he learned quickly. He was able to skip the 11th grade and to finish as the class valedictorian. He then started the University of Illinois (as an Engineering Physics major), switched to Mathematics and earned an accelerated Master's degree. From Illinois he moved to the graduate program at Princeton, where he wrote a thesis under the direction of Timothy O'Meara.
Han graduated from Princeton with a Ph.D. in 1959, but stayed on one more year as an Instructor. From 1960 to 1963 he served as a Benjamin Peirce Instructor at Harvard. He continued in the research direction of his thesis, mainly on finite groups and on quadratic forms over fields of characteristic 2. Together with R. Brauer, he edited the proceedings of one of the most influential meetings in the 1960s, which helped set the classification program of finite simple groups on the road to completion. While he was at Harvard he developed an introductory course in Abstract Algebra; his notes became a textbook with the same title that was published in 1967 by Academic Press and is still a standard reference.
In 1963 Han joined the University of Pennsylvania faculty. There he collaborated with Oscar Goldman on locally compact rings. Their joint work became very well known and led to a number of interesting results and new tools such as the Goldman-Sah product. Also, in his work at Pennsylvania with Leonard Charlap on the classification of flat Riemannian manifolds, he began his study of the homology and cohomology of groups, topics that would preoccupy him for the rest of his career.
In 1970 Han came to Stony Brook, where he stayed. He continued his research in finite groups and group cohomology but also began work on applications of group cohomology to Hilbert's Third Problem, about ``scissors congruence'' of polytopes in Euclidean, spherical, or hyperbolic $n$-space, a field in which he was to become the world's main authority. Much of this work was in a collaboration with Johan Dupont that atarted then and lasted the rest of his life.
Two polytopes are called ``scissors congruent" if they can be cut into the same finite number of subpolytopes such that the pieces of the two polytopes are pairwise congruent by means of isometries of the geometry in question. The problem seems part of classical geometry, but today, thanks to the efforts of Han and his collaborators, this area of research is fully integrated into modern mathematics, having close connections to well established fields such as homological algebra and algebraic $K$-theory, characteristic classes for flat bundles and foliations, hyperbolic 3-manifolds, and even (though more speculatively) subjects like motivic cohomology and conformal field theory.
At Stony Brook Han began working with members of the Institute for Theoretical Physics (now the C. N. Yang Institute) on the theory of exactly solvable models, and published joint work with Barry McCoy, Jacques Perk, and Shuang Tang, and with Eduardo Ramos and Robert Shrock. Towards the end of his life his various interests came together: he and Dupont published a paper in Communications in Mathematical Physics on ``Dilogarithm identities in conformal field theory and in group homology.'' Han actually worked on a wide range of problems in all kinds of mathematics extending into physics and chemistry, including an early paper on the structure of the exotic Carbon molecules of the fullerene family.
As Han said himself, ``I never lost my interest in science ...'' Although he loved ``useless'' mathematics, he was very happy when it could be used in elucidating problems in physics and chemistry. One of his main concerns about the education of mathematics majors was that they were not getting the solid exposure to science courses that would allow them the satisfaction of applying their mathematical knowledge to ``real-life'' situations, and also allow them access to the vast source of mathematical problems and phenomena encoded in the physical world.
Chih-Han Sah was always interested in problems of mathematics education, and became increasingly involved during his years at Stony Brook.
In the early 1990s, Sah became involved in the Gelfand project, an adaptation of the Russian mathematics correspondence schools to the United States. These correspondence schools had been very successful in the Soviet Union, but the transition to the U.S. needed some understanding of American students. Han helped with this. He also worked on a set of notes, intended for teachers, that revealed the several levels of meaning in Gelfand's assignments (and that unfortunately remains incomplete).
Han was a thoughtful critic of reform in both calculus and K-12 education. His contributions here are difficult to document, taking the form of memos, e-mail, and phone conversations rather than position papers or public talks. But he was the best kind of critic, the kind who, for example, would look up student records to see what actually happened rather than relying on his own notion of what he had expected.
The most public of his involvements in education was the e-mail list ``mathed" he established in August 1993. This grew out of a series of e-mail conversations with Hung-Hsi Wu and Richard Askey, who shared his concern. Han managed the list himself, and was careful to include people whose points of view were bound to differ from his own. He took great pains to keep the discussion civil as well as passionate. As Hung-Hsi Wu writes, ``Ultimately time may prove that the way Han handled the mathed group was his greatest contribution to education."