A professor tells his class that he will give them a surprise examination sometime next week.

For our purposes, an examination is considered a "surprise" if on that day, the class is not 100% sure that they will get an examination. (If they only suspect it, the examination can still be a surprise.)

Having given the class this warning, is it still possible for the professor to give them a surprise examination next week? (Assume that he will not give them an exam if it will not be a surprise.)

Chapter 43, The Unexpected Hanging, The Colossal Book of Mathematics, Martin Gardner, Norton Press, 2001. This is considered a real paradox and his writeup and history, along with the rest of the "colossal" book, is fascinating!

Posted by Marc Meyer on 2003-10-31 12:13:25