Albert Einstein (1879-1955)
Autograph letter signed ('A. Einstein') to Cornelius Lanczos, n.p., 9 October 1937
In German. One page, 280 x 216mm, including four mathematical equations. Provenance: Sotheby's, 26 November 1980, lot 160.

A critique of Lanczos's work on gravitational equations, using the 'vacuum field equation' R_ik=0. 'It makes no sense to postulate the equation R_ik=0...alongside your equation' (writing out the equation in question): Einstein points out that if one equation is fulfilled, the other necessarily is, and provides the reasoning, beginning with an equation including the term 'L' which represents 'a linear-homogenous Function'; if one performs a Bianchi operation on this, a further equation is produced, which proves Einstein's argument. The first equation therefore 'only makes sense on its own, i.e. without postulating R_ik=O. This is in fact a generalisation of the old gravitational equations. However, it does not seem that this generalisation is an essential generalisation', as the necessary identities between the initial equations are lacking.