Although they're not really "equations", I'd have liked to see some results from computation theory in this list, because they're often deep and beautiful without necessarily being difficult to formulate or understand. They also tend to be informative in a useful way rather than feeling too abstract to be relevant.
For example: the halting problem. Isn't it interesting that you can't write a computer program which can decide (in general) whether another program terminates? The proof is simple and it gives you a real tool to help you avoid trying to solve impossible problems. It's good to know the limits of your craft.
For example: the halting problem. Isn't it interesting that you can't write a computer program which can decide (in general) whether another program terminates? The proof is simple and it gives you a real tool to help you avoid trying to solve impossible problems. It's good to know the limits of your craft.