I am now occupied exclusively with the gravitational problem, and believe that I can overcome all difficulties with the help of a local mathematician friend. But one thing is certain, never before in my life have I troubled myself over anything so much, and that I have gained great respect for mathematics, whose more subtle parts I considered until now, in my ignorance, as pure luxury! ...She writes about his refusal to credit another long-time friend:
With this task in mind, in 1912, I was looking for my old student friend Marcel Grossmann, who had meanwhile become a professor of mathematics in the Swiss Federal Polytechnic institute. He was immediately caught in the fire, even though he had as a real mathematician a somewhat skeptical attitude towards physics. ...
He looked through the literature, and soon discovered that the particular implied mathematical problem was already solved by Riemann, Ricci and Levi-Civita. ...
Grossman will never claim to be considered a co-discoverer. He only helped in guiding me through the mathematical literature, but contributed nothing of substance to the results.
During a visit by Besso to Einstein in 1913 they both tried to solve the Einstein-Grossmann field equations to find the perihelion advance of Mercury. The theory predicted a wrong perihelion advance.This is not quite right, as this "correct precession" was derived from Grossmann's 1913 equation that empty space is Ricci-flat. The Mercury precession is regarded as Einstein's most original contribution to general relativity, but only because he refused to credit his friends.
Towards the end of 1915 Einstein abandoned the Einstein-Grossmann theory, and with his new General Theory of Relativity got the correct precession so quickly because he was able to apply the methods he had already worked out two years earlier with Besso. Einstein did not mention Besso, probably because he still considered him as a sounding board; this as opposed to his other friend, Grossmann, who was his active partner since 1912 in creating the Einstein-Grossmann theory.
She also explains how Einstein used words to exaggerate the originality of his work:
Alberto Martínez asks in his latest book Kinematics, "Was the formulation of the special theory of relativity a discovery [Entdeckung] or an invention [Erfindung]? Nowadays, many writers call it a 'discovery'. But throughout his life, Einstein emphasized the importance of invention, when characterizing his theoreticalYes, I think that most people would say that non-Euclidean geometry, and its application to relativity, was a discovery, not an invention. Einstein did not discover or invent it. He was a big egomaniac to claim all the credit that he did.