Einstein refers to Poincaré in connection with the inertia of energy in 1906 and the non-Euclidean geometry in 1921, but not in connection with the Lorentz transformation, the relativity principle or the synchronization procedure by light signals. However, in the last years before his death Einstein acknowledged some of Poincaré's contributions (according to Darrigol, maybe because his biographer Pais in 1950 sent Einstein a copy of Poincarè's Palermo paper, which he said that he had not read before). Einstein wrote in 1953: