Euler proposed that the equation
a4+b4+c4 =d4
had no solutions in integers (excluding the trivial 0,0,0 of course).
Nowadays, is it true or not?
No, not true today.
For instance, consider (0,0,1).
0^4 + 0^4 + 1^4 = 1^4.
Somehow, I do not think this is the answer the puzzle author is looking for.