where (·, ·) refers to the bilinear form of signature (1, 3) on R exposed by the right hand side formula in (D3). The alternative notation defined on the right is referred to as the relativistic dot product. Spacetime mathematically viewed as R endowed with this bilinear form is known as Minkowski space M. The Lorentz transformation is thus an element of the group O(1, 3), the Lorentz group or, for those that prefer the other metric signature, O(3, 1) (also called the Lorentz group). One has: