A letter written by Albert Einstein containing his E=mc² equation has sold at auction in the US for more than $1.2m (£850,000) - three times more than had been expected.
Experts say there are only three other known examples of the equation in the physicist's handwriting.
The equation was first published in a scientific paper by Einstein in 1905.
It explains the interchangeability of energy and mass.
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.
The letter was part of Silberstein's personal archives, which were sold by his descendants, according to the Associated Press news agency. It reported the buyer was identified only as an anonymous document collector.
Correction: The original version of this article incorrectly said: "It explains the interchangeability of energy and mass when a body moves at the speed of light." It has been amended.