A pair of handwritten, signed letters by Albert Einstein will be sold at RR Auction on April 13.
Both are addressed to fellow physicist Dr Arthur Schidlof on the subject of photochemical equivalence, and date to the summer of 1913.
Einstein wrote the letter
Einstein's research into the subject culminated in the publication of his Emission and Absorption of Radiation in Quantum Theory (1916).
His work on this question was an important waypoint on the road to his general theory of relativity, which was also published in 1916.
Letters written by Einstein tend to sell very well at auction, although the top sums are reserved for those that state his views on religion.
In 2015 a letter addressing his views on the subject sold for $425,000, while another on God made $75,000.
The present lot is expected to realise around $80,000-100,000.
An autograph from Barack Obama (as president) dedicated to his descendent village is also on offer, with an estimate of $10,000-15,000.
It's inscribed: "To the People of K'Obama—Thank you for your prayers and support! Barack Obama."
Kogelo, as the village is also known, is located near Lake Victoria in Kenya. Obama has visited several times, but only once as president.
The record for an Obama signature is $20,026, set for an autographed 2012 NCAA basketball leaderboard at Nate D Sanders in 2013.
He has signed very few items during his tenure, making his autograph the most valuable of any living US president.
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