A problem in the theory of algebraic invariants that was solved by Hilbert using an existence proof.
Gordan's Problem
References
Derbyshire, J. Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics. New York: Penguin, pp. 184-185, 2004.Cite this as:
Weisstein, Eric W. "Gordan's Problem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/GordansProblem.html