Supertropical quadratic forms II: Tropical trigonometry and applications

Abstract

This paper is a sequel of [Z. Izhakian, M. Knebusch and L. Rowen, Supertropical semirings and supervaluations, J. Pure Appl. Algebra 220(1) (2016) 61–93], where we introduced quadratic forms on a module [Formula: see text] over a supertropical semiring [Formula: see text] and analyzed the set of bilinear companions of a quadratic form [Formula: see text] in case the module [Formula: see text] is free, with fairly complete results if [Formula: see text] is a supersemifield. Given such a companion [Formula: see text], we now classify the pairs of vectors in [Formula: see text] in terms of [Formula: see text] This amounts to a kind of tropical trigonometry with a sharp distinction between the cases for which a sort of Cauchy–Schwarz (CS) inequality holds or fails. This distinction is governed by the so-called CS-ratio [Formula: see text] of a pair of anisotropic vectors [Formula: see text] in [Formula: see text]. We apply this to study the supertropicalizations (cf. [Z. Izhakian, M. Knebusch and L. Rowen, Supertropical semirings and supervaluations, J. Pure Appl. Algebra 220(1) (2016) 61–93]) of a quadratic form on a free module [Formula: see text] over a field in the simplest cases of interest where [Formula: see text]. In the last part of the paper, we introduce a suitable equivalence relation on [Formula: see text], whose classes we call rays. (It is coarser than usual projective equivalence.) For anisotropic [Formula: see text] the CS-ratio [Formula: see text] depends only on the rays of [Formula: see text] and [Formula: see text]. We develop essential basics for a kind of convex geometry on the ray-space of [Formula: see text], where the CS-ratios play a major role.