  The intermediate stage of the development of general relativity is
inseparable of Marcel Grossmann's mathematical assistance. Einstein
acknowledges Grossmann's help during 1912-1914 to the development of general
relativity. In fact, as with special relativity so was it with General
relativity, Einstein received assistance only from his old friends, Marcel
Grossmann and Michele Besso. However, he continued to consider Besso as his
eternal "sounding board"...
