A new method to detect projective equivalences and symmetries of rational $3D$ curves

Abstract: We present a new approach using differential invariants to detect projective equivalences and symmetries between two rational parametric $3D$ curves properly parametrized. In order to do this, we introduce two differential invariants that commute with M\"obius transformations, which are the transformations in the parameter space associated with the projective equivalences between the curves. The M\"obius transformations are found by first computing the gcd of two polynomials built from the differential invariants, and then searching for the M\"obius-like factors of this gcd. The projective equivalences themselves are easily computed from the M\"obius transformations. In particular, and unlike previous approaches, we avoid solving big polynomial systems. The algorithm has been implemented in Maple, and evidences of its efficiency as well as a comparison with previous approaches are given.