In 1943 a girl named Barbara from Washington D.C. wrote Einstein:
"I'm a little below average in mathematics".
She told Einstein that she had to work at it harder than most of her friends.
Replying in English from Princeton on January 7, 1943, Einstein wrote in part as follows:
"Do not worry about your difficulties in mathematics; I can assure you that mine are still greater".
The following example represents Einstein’s difficulties in mathematics:
In 1917 Willem de Sitter suggested a solution to Einstein’s field equations with the cosmological constant, an empty world. In the static form of the de Sitter solution, there is a singularity at the equator. Hence, At de Sitter’s world's equator, it takes an infinite time for anything to happen.
Einstein, however, found it hard to give up Mach's principles and he suggested instead that de Sitter's model was actually not-matter free. In March 1918 Einstein was quite sure that something in de Sitter’s line element might be badly mistaken. He thus searched for matter in de Sitter's universe!
According to general relativity, the closer clocks are to a material source, the more slowly they run (gravitational time dilation). Einstein thus reasoned that clocks slowed down as they approached the singularity at the equator of de Sitter’s world. Einstein concluded that all matter in de Sitter's world was concentrated in this intrinsic singularity at the equator and therefore, after all, de Sitter’s world was not empty!
De Sitter explained to Einstein that this singularity was an artifact of the static form but Einstein was as stubborn as a mule and wasn’t even willing to listen! He was hundred percent sure that no choice of coordinates could ever remove this singularity!
Finally in June 1918 the great mathematician Felix Klein wrote to Einstein:
To which Einstein replied:
The Collected Papers of Albert Einstein, Vol. 8.