A self-working card curiosity by Shippensburg University mathematician Douglas E. Ensley:
I give you the four aces from a deck of cards and turn my back. Then I ask you to stack the four cards face up with the heart at the bottom, then the club, the diamond, and the spade. Now turn the uppermost card, the spade, face down.
Now you’re invited to perform any of these operations as many times and in any order that you wish:
- Cut any number of cards from the top of the stack to the bottom.
- Turn the top two cards over as one.
- Turn the entire stack over.
When you’ve finished, I ask you to turn the topmost card over, then turn the top two cards over as one, then turn the top three cards over as one. I predict that the club is the only card facing the opposite way from the others, and as long as you’ve followed the directions above, it always will be.
The answer is explained by group theory — see the article below for the details.
(Douglas E. Ensley, “Invariants Under Group Actions to Amaze Your Friends,” Mathematics Magazine 72:5 [December 1999], 383-387.)