Where did Karl Schwarzschild derived his solution? Does anyone know more about circumstances of Karl Schwarzschild at the Russian front in 1915 where he allegedly derived his famous solution of the Einstein equations (describing a black hole)? Sources I know only repeat that he served in artillery, but do not mention the exact place where he wrote his letter to Einstein including the solution, or other details.
 A: Googling for "Einstein-Schwarzschild correspondence" (since the solution first appeared in a letter to Einstein), I came across a valuable source: a new book A New Approach to Differential Geometry Using Clifford's Geometric Algebra by John Snygg. Some relevant pages (unfortunately without pagination) are freely available at Google Books. 
Snygg writes (in my transcription from the Google Books images):
"In August of 1915, Karl Schwarzschild along with his artillery brigade 
was assigned to the Tenth Army on the Russian Front at Kovoso in the 
present day Lithuania [...]".
I have not been able to localize Kovoso by now, but this doesn't matter 
as you will see:
"According to most biographies of Einstein, Schwarzschild sent his 
famous solution to Einstein from the Russian front in a letter dated 
December 22, 1915. However, correspondence with his wife shows that by 
the end of September he had been relocated to Mulhouse in Alsace. He was 
then relocated again to someplace else but by December 1, he was back in 
Mulhouse. Historian Tilman Sauer has drawn my attention to a letter 
Schwarzschild wrote to Arnold Sommerfeld on the same date that 
Schwarzschild mailed his solution to Einstein [Schwarzschild 1915]. In 
the letter to Sommerfeld Schwarzschild describes hearing canon fire from 
Hartmannweilerkopf, which is about 10 km from Mulhouse."
