Discrete Painlev\'e equations and pencils of quadrics in P³

Discrete Painlev\'e equations constitute a famous class of integrable non-autonomous second order difference equations. A classification scheme proposed by Sakai interprets a discrete Painlev\'e equation as a birational map between generalized Halphen surfaces (surfaces obtained from P¹× P¹ by blowing up at eight points). Sakai's classification is thus based on the classification of generalized Halphen surfaces. We propose a novel geometric interpretation of discrete Painlev\'e equations, where the family of generalized Halphen surfaces is replaced by a pencil of quadrics in P³. A discrete Painlev\'e equation is viewed as a birational transformation of P³ that preserves the pencil and maps each quadric of the pencil to a different one, according to a M\"obius transformation of the pencil parameter. Thus, our scheme is based on the classification of pencils of quadrics in P³. While historically discrete Painlev\'e equations appeared as de-autonomizations of QRT maps, in our scheme they are viewed as deformations of 3D QRT maps introduced in our previous paper. A 3D QRT map is defined geometrically as a composition of involutions along generators of quadrics of a pencil, preserving the intersection curves with a second pencil of quadrics. The base set of the net of quadrics spanned by both pencils consists of eight points (which play the role of the eight blow-up points of generalized Halphen surfaces). A Painlev\'e deformation of a 3D QRT map is obtained by composing involutions along generators with a birational (often linear) transformation of P³ under which the pencil remains invariant, but the individual quadrics are mapped according to a M\"obius transformation of the pencil parameter.