BOSTON—A letter written by Albert Einstein in which he writes out his famous E = mc2 equation has sold at auction for more than $1.2 million, about three times more than it was expected to get, Boston-based RR Auction said Friday. Archivists at the Einstein Papers Project at the California Institute of Technology and the Hebrew University of Jerusalem say there are only three other known examples of Einstein writing the famous equation in his own hand. This fourth example, the only one in a private collection, only became public recently, according to RR Auction, which had expected it to sell for about $400,000. “It’s an important letter from both a holographic and a physics point of view,” Bobby Livingston, executive vice president at RR Auction said, calling the equation the most famous in the world. The equation—energy equals mass times the speed of light squared—changed physics by demonstrating that …