Characterizations of Ruled Surfaces in $\mathbb{R}^3$ and of   Hyperquadrics in $\mathbb{R}^{n+1}$ via Relative Geometric Invariants
  We consider hypersurfaces in the real Euclidean space $\mathbb{R}^{n+1}$ ($n\geq2$) which are relatively normalized. We give necessary and sufficient conditions a) for a surface of negative Gaussian curvature in $\mathbb{R}^3$ to be ruled, b) for a hypersurface of positive Gaussian curvature in $\mathbb{R}^{n+1}$ to be a hyperquadric and c) for a relative normalization to be constantly proportional to the equiaffine normalization. 
Stylianos Stamatakis, Ioannis Kaffas and Ioanna-Iris Papadopoulou
math.DG