Einstein equation at singularities
  Einstein's equation is rewritten in an equivalent form, which remains valid
at the singularities in some major cases. These cases include the Schwarzschild
singularity, the Friedmann-Lema\^itre-Robertson-Walker Big Bang singularity,
isotropic singularities, and a class of warped product singularities. This
equation is constructed in terms of the Ricci part of the Riemann curvature (as
the Kulkarni-Nomizu product between Einstein's equation and the metric tensor).
