  This paper aims to revisit the mathematical foundations of both General
Relativity and Electromagnetism after one century, in the light of the formal
theory of systems of partial differential equations and Lie pseudogroups (D.C.
Spencer, 1970) or Algebraic Analysis, namely a mixture of differential geometry
and homological algebra (M. Kashiwara, 1970). Among the new results obtained,
we may quote: 1) In dimension 4 only, the 9 Bianchi identities that must be
satisfied by the 10 components of the Weyl tensor are described by a second
order operator and have thus nothing to do with the 20 first order Bianchi
identities for the 20 components of the Riemann tensor. This result, not known
after one century, has been recently confirmed by A. Quadrat (INRIA) using new
computer algebra packages. 2) The Ricci tensor R is a section of the Ricci
bundle of symmetric covariant 2-tensors which is the kernel of the canonical
projection of the Riemann bundle onto the Weyl bundle, induced by the canonical
inclusion of the classical Killing system (Poincare group) into the conformal
Killing system (Conformal group). It has only to do with the second order jets
(elations) of the conformal Killing system because any 1-form with value in the
bundle of elations can be decomposed in the direct sum (R,F) where the
electromagnetic field F is a section of the vector bundle of skewsymmetric
covariant 2-tensors. It follows therefore that electromagnetism and gravitation
have only to do with second order jets. 3) The 10 linearized second order
Einstein equations are parametrizing the 4 first order Cauchy stress equations
but cannot be parametrized themselves. As a byproduct of this negative result,
these 4 Cauchy stress equations have nothing to do with the 4 divergence-type
equations usually obtained from the 20 Bianchi identities by contraction of
indices.
