Abstract. This article is a survey of Nash's contributions to algebraic geometry, focusing on the topology of real algebraic sets and on arc spaces of singularities.Nash wrote two papers in algebraic geometry, one at the beginning of his career [Nash52] and one in 1968, after the onset of his long illness; the latter was published only much later [Nash95]. With these papers Nash was ahead of the times; it took at least 20 years before their importance was recognized. By now both are seen as starting points of significant directions within algebraic geometry. I had the privilege to work in these areas and discuss the problems with Nash. It was impressive that, even after 50 years, these questions were still fresh in his mind and he still had deep insights to share.Section 1 is devoted to the proof of the main theorem of [Nash52], and Section 3 is a short introduction to the study of arc spaces pioneered by [Nash95]. Both of these are quite elementary. Section 2 discusses subsequent work on a conjecture of [Nash52]; some familiarity with algebraic geometry is helpful in reading it.
The topology of real algebraic setsOne of the main questions occupying mathematicians around 1950 was to understand the relationships among various notions of manifolds.The Hauptvermutung (=Main Conjecture) formulated by Steinitz and Tietze in 1908 asserted that every topological manifold should be triangulable. The relationship between triangulations and differentiable structures was not yet clear. Whitney proved that every compact C 1 -manifold admits a differentiable-even a real analytic-structure [Whi36], but the groundbreaking examples of Milnor [Mil56,Mil61] were still in the future, and so were the embeddability and uniqueness of real analytic structures [Mor58,Gra58].Nash set out to investigate whether one can find even stronger structures on manifolds. It is quite likely that polynomials form the smallest class of functions that could conceivably be large enough to describe all manifolds. Definition 1. A real algebraic set is the common zero set of a collection of polynomialsComments. For the purposes of this article, one can just think of these as subsets of R N , though theoretically it is almost always better to view X = X(R) as the real points of the complex algebraic set X(C), which consist of all complex solutions of