We discuss geometric integrability of Hirota's discrete KP equation in the framework of projective geometry over division rings using the recently introduced notion of Desargues maps. We also present the Darboux-type transformations, and we review symmetries of the Desargues maps from the point of view of root lattices of type A and the action of the corresponding affine Weyl group. Such a point of view facilities to study the relation of Desargues maps and the discrete conjugate nets. Recent investigation of geometric integrability of Desargues maps allowed to introduce two maps satisfying functional pentagon equation. Moreover, the ultra-locality requirement imposed on the maps leads to Weyl commutation relations. We show that the pentagonal property of the maps allows to define a coproduct in the quantum plane bi-algebra, which can be extended to the corresponding Hopf algebra.The relevance of a geometric theorem is determined by what the theorem tells us about space, and not by the eventual difficulty of the proof. The Desargues' theorem of projective geometry comes as close as a proof can to the Zen ideal. It can be summarized in two words: "I see!" Nevertheless, Desargues' theorem, far from trivial despite the simplicity of its proof, has many more applications both in geometry and beyond ...
Gian Carlo Rota, The Phenomenology of Mathematical Proof
IntroductionEverybody interested in the (pre)history of soliton theory should consult monographs of Bianchi [8], Darboux [25,26], Eisenhart [47] or Tzitzéica [96]. In these geometry books, which summarize classical XIX-th century style developments in theory of submanifolds and their transformations, one can recognize many fundamental facts from the theory of integrable partial differential equations. In looking for analogous geometric interpretation of integrable partial difference systems we have found that very often their integrability features are encoded in incidence geometry theorems of Pappus, Desargues, Pascal, Miquel and others [21,31,32,34,36], compare also works [15,17,16,50,55,56,59,60,61] written in a similar spirit; for introduction to projective geometry and its subgeometries see [24,83].Hirota's discrete Kadomtsev-Petviashvili (KP) equation [52] may be considered as the Holy Grail of integrable systems theory, both on the classical and the quantum level [65]. In the present paper, based on our earlier publications [36,37,42], we review geometric aspects of the non-commutative Hirota system within the framework of projective geometry over division rings. The crucial notion here is that of Desargues maps, where the underlying geometric property is collinearity of three points, which gives the linear problem for the Hirota system. This should be considered as further simplification of (already rather non-complicated) approach to integrable discrete geometry via the theory of multidimensional quadrilateral lattices [39] based on coplanarity of four points. We remark that the quadrilateral lattice is the integrable discrete analogue [84,28] of the co...