A rare letter written by Albert Einstein in which he wrote down his renowned ‘E = mc²’ mass–energy equivalence formula has been sold at auction for a whopping $1.2 million after a furious bidding war, the Mail reported.

Boston-based RR Auctions, which handled the sale, announced that it was more than 3 times what was expected for the letter, which went on sale earlier this month.

Bobby Livingston, executive vice president at RR Auction said in an interview with the Associated Press: “It’s an important letter from both a holographic and a physics point of view.”

Initially, there were 5 bidders for the rare item, but once the price of the item hit $700,000, only 2 bidders remained.

The one who bought the Einstein letter has preferred to remain anonymous, RR Auctions said.

There are 4 known examples of the famous equation penned in Einstein’s own handwriting, but this was the only one in a private collection.

The auction began on May 13 and ended on May 20.

Einstein demonstrated mass-energy equivalence in 1905 – his so-called “miracle year” which also saw him publish ground-breaking papers whereby he introduced the theory of special relativity, explained Brownian motion and outlined the photoelectric effect.