Einstein's letter containing his famous equation E=mc² goes up for sale

A letter penned by Albert Einstein which is considered rare for containing his famous 'E=mc²' mass-energy equivalence formula has gone on sale. The letter, dated October 26, 1946, is one of only four known examples of the formula having been written in his hand. In the letter, Einstein responded to a query from Polish-American physicist Ludwik Silberstein.