Time traveler talks to a mathematician

The time traveler says, "Hello, in my grad school I have learned that it is impossible for any number which is a power greater than the second to be written as the sum of two like powers such as x^n + y^n = z^n for n > 2." "Show me how you proved it," the mathematician says. "Indeed! I studied it for my thesis." The time traveler, then, goes on with his proof. "Thank you, traveler, I wanted to take notes but right now I only have this book with a tiny margin."