It comes
from George Gamow's autobiography, "My World Line", Viking Press, 1970.
 
Pages 19-20 (and pardon my typos)
 
"Here is a story told to me by one of my friends who was at that time
a young professor of physics in Odessa. His name was Igor Tamm (Nobel
Prize laureate in Physics, 1958). Once when he arrived in a neighboring 
village, at that period when Odessa was occupied by the Reds, and was
negotiating with a villager as to how many chickens he could get for
half a dozen silver spoons, the village was captured by one of the
Makhno bands, who were roaming the country, harassing the Reds. Seeing
his city clothes (or what was left of them), the capturers [sic]
brought him to the Ataman, a bearded fellow in a tall black fur 
hat with machine-gun cartridge ribbons crossed on his broad chest and
a couple of hand grenades hanging on the belt.
 
'You son-of-a-bitch, you Communist agitator, undermining our Mother 
Ukraine! The punishment is death.'
 
'But no,' answered Tamm, 'I am a professor at the University of Odessa 
and have come here only to get some food.'
 
'Rubbish!' retorted the leader. 'What kind of professor are you ?'
 
'I teach mathematics.'
 
'Mathematics?' said the Ataman. 'All right! Then give me an estimate of
the error one makes by cutting off Maclaurin's series at the nth term.
Do this, and you will go free. Fail, and you will be shot!'
 
Tamm could not believe his ears, since this problem belongs to a rather 
special branch of higher mathematics. With a shaking hand, and under
the muzzle of the gun, he managed to work out the solution and handed
it to the Ataman.
 
'Correct!' said the Ataman. 'Now I see that you really are a professor. 
Go home!'
 
Who was this man? No one will ever know. If he was not killed later, he 
may well be lecturing now on higher mathematics in some Ukrainian
university."
 
I tell this story every other year or so to my physics students when
they cannot be bothered to remember the form of the remainder in Taylor
expansions....
 
David Derbes