"Untitled joke" joke
How many mathematicians does it take to screw in a light bulb?
In earlier work, Wiener  has shown that one mathematician can change a light bulb.
If k mathematicians can change a light bulb, and if one more simply watches them do it, then k+1 mathematicians will have changed the light bulb. Therefore, by induction, for all n in the positive integers, n mathematicians can change a light bulb.
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