A letter written by Albert Einstein containing his most famous equation, E=mc2, has been sold at an auction in the US for over $1.2m (£850,000).
Experts say there are only three other known examples of the equation in the physicist's handwriting, reports BBC.
The equation was first published in a scientific paper by Einstein in 1905.
It explains the interchangeability of energy and mass when a body moves at the speed of light.
The equation - energy equals mass times the speed of light squared - is a fundamental concept in modern physics.
This was the only example of the equation in a private collection and only became public recently, said Boston-based RR Auction, which sold the letter.
"[It's] an important letter from both a holographic and a physics point of view," RR Auction said in a statement, calling it "the most well-known equation ever set forth".
The one-page letter in German is dated 26 October 1946 and addressed to Polish-American physicist Ludwik Silberstein, who challenged some of Einstein's theories.
According to the Associated Press, the buyer was identified only as an anonymous document collector.