An algebraic geometry of paths via the iterated-integrals signature

Abstract: Contrary to previous approaches bringing together algebraic geometry and signatures of paths, we introduce a Zariski topology on the space of paths itself, and study path varieties consisting of all paths whose iterated-integrals signature satisfies certain polynomial equations. Particular emphasis lies on the role of the non-associative halfshuffle, which makes it possible to describe varieties of paths that satisfy certain relations all along their trajectory. Specifically, we may understand the set of paths on a given classical algebraic variety in $\mathbb{R}^d$ starting from a fixed point as a path variety, e.g. paths on a sphere. While the characteristic geometric property of halfshuffle varieties is that they are stable under stopping paths at an earlier time, we furthermore study varieties which are stable under the natural (semi)group operation of concantenation of paths. We point out how the notion of dimension for path varieties crucially depends on the fact that they may be reducible into countably infinitely many subvarieties. Finally, we see that studying halfshuffle varieties naturally leads to a generalization of classical algebraic curves, surfaces and affine varieties in finite dimensional space. These generalized algebraic sets, an example being the graph of the exponential function, are now described through iterated-integral equations. Keywords: path variety; shuffle ideal; halfshuffle; deconcatenation coproduct; tensor algebra; Zariski topology; concatenation of paths; Chen's identity; subpath; tree-like equivalence; regular map; generalized variety