Leyland numbers (named for Paul Leyland) are positive integers of the form $x^y + y^x$ , where x and y are naturals > 1, and also the number 3. The OEIS link is https://oeis.org/A076980
 
I thought that this might have some nice bunch of Pythagorean triples generated but I was wrong. In the first 5000 such numbers from https://oeis.org/A076980/b076980.txt apparently the only square is $100$.

Are there any other Leyland numbers that are squares? And why not? What sort of mathematics should I study to understand this?

Thank you