3 The notion of Levi-Civita connection is recalled in Section 4. 4 Of course, if the tensor is not positive definite, it does not induce a distance on M , and in particular there is no notion of "minimization of distance" for the geodesics. 5 A smooth manifold is pseudo-Riemannian if it is endowed with a non-degenerate smooth (0, 2)-tensor. If the tensor is positive definite at each point, then the manifold is Riemannian. If the tensor has a negative direction, but no more than one linearly independent negative direction at each point, then the manifold is Lorentzian. 6 The isometry group of M is bigger than O(p, q) if M is not connected. 7 This follows from the Gauss formula and the fact that the shape operator on M is ± the identity. The fact that the sectional curvature is constant also follows from the preceding item.It is obvious that the projective quotient of M is anti-isometric to M, and will thus be denoted by M. Moreover, in the case where M is not connected, it has two connected components, which correspond under the antipodal map x → −x. In particular, M is connected.By construction, M is a subset of the projective space RP n . Indeed, topologically M can be defined asor the same definition with > replaced by <, depending on the case. Hence any choice of an affine hyperplane in R n+1 which does not contain the origin will give an affine chart of the projective space, and the image of M in the affine chart will be an open subset of an affine space of dimension n.Recall that PGL(n + 1, R), the group of projective transformations (or homographies), is the quotient of GL(n + 1, R) by the non-zero scalar transformations. Even if M is not connected, an isometry of M passes to the quotient only if it is an element of O(p, q). Hence Isom(M), the isometry group of M, is PO(p, q), the quotient of O(p, q) by {±Id} (indeed elements of O(p, q) have determinant equal to 1 or −1).
Lines and pseudo-distancePseudo-distance on M. A geodesic c of M is the non-empty intersection of M with a linear plane. This intersection might not be connected. The geodesic c of M is said to be