# Alexander Grothendieck : biography

In 1956, he applied the same thinking to the Riemann–Roch theorem, which had already recently been generalized to any dimension by Hirzebruch. The Grothendieck–Riemann–Roch theorem was announced by Grothendieck at the initial Mathematische Arbeitstagung in Bonn, in 1957. It appeared in print in a paper written by Armand Borel with Serre. This result was his first major achievement in algebraic geometry. He went on to plan and execute a major foundational programme for rebuilding the foundations of algebraic geometry, which were then in a state of flux and under discussion in Claude Chevalley’s seminar; he outlined his programme in his talk at the 1958 International Congress of Mathematicians.

His foundational work on algebraic geometry is at a higher level of abstraction than all prior versions. He adapted the use of non-closed generic points, which led to the theory of schemes. He also pioneered the systematic use of nilpotents. As ‘functions’ these can take only the value 0, but they carry infinitesimal information, in purely algebraic settings. His *theory of schemes* has become established as the best universal foundation for this major field, because of its great expressive power as well as technical depth. In that setting one can use birational geometry, techniques from number theory, Galois theory and commutative algebra, and close analogues of the methods of algebraic topology, all in an integrated way.See, for example, .

He is also noted for his mastery of abstract approaches to mathematics and his perfectionism in matters of formulation and presentation. Relatively little of his work after 1960 was published by the conventional route of the learned journal, circulating initially in duplicated volumes of seminar notes; his influence was to a considerable extent personal. His influence spilled over into many other branches of mathematics, for example the contemporary theory of D-modules. (It also provoked adverse reactions, with many mathematicians seeking out more concrete areas and problems.)

### EGA and SGA

The bulk of Grothendieck’s published work is collected in the monumental, and yet incomplete, *Éléments de géométrie algébrique* (EGA) and *Séminaire de géométrie algébrique* (SGA). The collection *Fondements de la Géometrie Algébrique* (FGA), which gathers together talks given in the Séminaire Bourbaki, also contains important material.

Perhaps Grothendieck’s deepest single accomplishment is the invention of the étale and l-adic cohomology theories, which explain an observation of André Weil’s that there is a deep connection between the topological characteristics of a variety and its diophantine (number theoretic) properties. For example, the number of solutions of an equation over a finite field reflects the topological nature of its solutions over the complex numbers. Weil realized that to prove such a connection one needed a new cohomology theory, but neither he nor any other expert saw how to do this until such a theory was found by Grothendieck.

This program culminated in the proofs of the Weil conjectures, the last of which was settled by Grothendieck’s student Pierre Deligne in the early 1970s after Grothendieck had largely withdrawn from mathematics.

### Major mathematical topics (from *Récoltes et Semailles*)

He wrote a retrospective assessment of his mathematical work (see the external link *La Vision* below). As his main mathematical achievements ("maître-thèmes"), he chose this collection of 12 topics (his chronological order):

- Topological tensor products and nuclear spaces
- "Continuous" and "discrete" duality (derived categories and "six operations").
*Yoga*of the Grothendieck–Riemann–Roch theorem (K-theory, relation with intersection theory).- Schemes.
- Topoi.
- Étale cohomology including l-adic cohomology.
- Motives and the motivic Galois group (and Grothendieck categories)
- Crystals and crystalline cohomology,
*yoga*of De Rham and Hodge coefficients. - Topological algebra, infinity-stacks, ‘dérivateurs’, cohomological formalism of toposes as an inspiration for a new homotopic algebra
- Tame topology.
*Yoga*of anabelian geometry and Galois–Teichmüller theory.- Schematic point of view, or "arithmetics" for regular polyhedra and regular configurations of all sorts.

He wrote that the central theme of the topics above is that of topos theory, while the first and last were of the least importance to him.

Here the term *yoga* denotes a kind of "meta-theory" that can be used heuristically; Michel Raynaud writes the other terms "Ariadne’s thread" and "philosophy" as effective equivalents.