In 1957, after De Giorgi had published the solution to Hilbert’s nineteenth problem, mathematicians all over the world got busy trying to find all possible extensions to it. After half a century, the field of studies devoted to the problem had flourished. But there was still an open question. The nineteenth problem, as defined by Hilbert, and solved by De Giorgi and Nash, was valid for a single differential equation. Now, the question was whether it was possible to extend the problem to systems of equations, that is, to groups of differential equations.
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