Hilbert's nineteenth problem

Hilbert's nineteenth problem is one of the 23 Hilbert problems, set out in a list compiled by David Hilbert in 1900. It asks whether the solutions of regular problems in the calculus of variations are always analytic. Informally, and perhaps less directly, since Hilbert's concept of a "''regular variational problem''" identifies this precisely as a variational problem whose Euler–Lagrange equation is an elliptic partial differential equation with analytic coefficients, Hilbert's nineteenth problem, despite its seemingly technical statement, simply asks whether, in this class of partial differential equations, any solution inherits the relatively simple and well understood property of being an analytic function from the equation it satisfies. Hilbert's nineteenth problem was solved independently in the late 1950s by Ennio De Giorgi and John Forbes Nash, Jr.

History

The origins of the problem

David Hilbert presented what is now called his nineteenth problem in his speech at the second International Congress of Mathematicians. In he states that, in his opinion, one of the most remarkable facts of the theory of analytic functions is that there exist classes of partial differential equations which admit only analytic functions as solutions, listing Laplace's equation, Liouville's equation, the minimal surface equation and a class of linear partial differential equations studied by Émile Picard as examples. He then notes that most partial differential equations sharing this property are Euler–Lagrange equations of a well defined kind of variational problem, satisfying the following three properties:See .
: = \text \qquad
\left \frac=p \quad;\quad \frac=q \right/math>,
:$\frac\cdot\frac - \left(\frac\right)^2 > 0$,
: is an analytic function of all its arguments and .
Hilbert calls this a "''regular variational problem''". Property means that these are minimum problems. Property is the ellipticity condition on the Euler–Lagrange equations associated to the given functional, while property is a simple regularity assumption about the function . Having identified the class of problems considered, he poses the following question: "''... does every Lagrangian partial differential equation of a regular variation problem have the property of admitting analytic integrals exclusively?''" He asks further if this is the case even when the function is required to assume boundary values that are continuous, but not analytic, as happens for Dirichlet's problem for the potential function .

The path to the complete solution

Hilbert stated his nineteenth problem as a regularity problem for a class of elliptic partial differential equation with analytic coefficients. Therefore the first efforts of researchers who sought to solve it were aimed at studying the regularity of classical solutions for equations belonging to this class. For solutions, Hilbert's problem was answered positively by in his thesis. He showed that solutions of nonlinear elliptic analytic equations in two variables are analytic. Bernstein's result was improved over the years by several authors, such as , who reduced the differentiability requirements on the solution needed to prove that it is analytic. On the other hand, direct methods in the calculus of variations showed the existence of solutions with very weak differentiability properties. For many years there was a gap between these results. The solutions that could be constructed were known to have square integrable second derivatives, but this was not quite strong enough to feed into the machinery that could prove they were analytic, which needed continuity of first derivatives. This gap was filled independently by , and , who were able to show the solutions had first derivatives that were Hölder continuous. By previous results this implied that the solutions are analytic whenever the differential equation has analytic coefficients, thus completing the solution of Hilbert's nineteenth problem. Subsequently, Jürgen Moser gave an alternate proof of the results obtained by , and .

Counterexamples to various generalizations of the problem

The affirmative answer to Hilbert's nineteenth problem given by Ennio De Giorgi and John Forbes Nash raised the question if the same conclusion holds also for Euler–Lagrange equations of more general functionals. At the end of the 1960s, , and independently constructed several counterexamples, showing that in general there is no hope of proving such regularity results without adding further hypotheses.

Precisely, gave several counterexamples involving a single elliptic equation of order greater than two with analytic coefficients. For experts, the fact that such equations could have nonanalytic and even nonsmooth solutions created a sensation.

and gave counterexamples showing that in the case when the solution is vector-valued rather than scalar-valued, it need not be analytic; the example of De Giorgi consists of an elliptic system with bounded coefficients, while the one of Giusti and Miranda has analytic coefficients. Later, provided other, more refined, examples for the vector valued problem.For more information about the work of Jindřich Nečas see the work of and .

De Giorgi's theorem

The key theorem proved by De Giorgi is an a priori estimate stating that if ''u'' is a solution of a suitable linear second order strictly elliptic PDE of the form
:$D_i(a^(x)\,D_ju)=0$
and $u$ has square integrable first derivatives, then $u$ is Hölder continuous.

Application of De Giorgi's theorem to Hilbert's problem

Hilbert's problem asks whether the minimizers $w$ of an energy functional such as
:$\int_UL(Dw)\,\mathrmx$
are analytic. Here $w$ is a function on some compact set $U$ of R^''n'', $Dw$ is its gradient vector, and $L$ is the Lagrangian, a function of the derivatives of $w$ that satisfies certain growth, smoothness, and convexity conditions. The smoothness of $w$ can be shown using De Giorgi's theorem
as follows. The Euler–Lagrange equation for this variational problem is the non-linear equation
:$\sum\limits_^n(L_(Dw))_ = 0$
and differentiating this with respect to $x_k$ gives
:$\sum\limits_^n(L_(Dw)w_)_ = 0$
This means that $u=w_$ satisfies the linear equation
:$D_i(a^(x)D_ju)=0$
with
:$a^ = L_(Dw)$
so by De Giorgi's result the solution ''w'' has Hölder continuous first derivatives, provided the matrix $L_$ is bounded. When this is not the case, a further step is needed: one must prove that the solution $w$ is Lipschitz continuous, i.e. the gradient $Dw$ is an $L^\infty$ function.

Once ''w'' is known to have Hölder continuous (''n''+1)st derivatives for some ''n'' ≥ 1, then the coefficients ''a''^''ij'' have Hölder continuous ''n''th derivatives, so a theorem of Schauder implies that the (''n''+2)nd derivatives are also Hölder continuous, so repeating this infinitely often shows that the solution ''w'' is smooth.

Nash's theorem

John Nash gave a continuity estimate for solutions of the parabolic equation
:$D_i(a^(x)D_ju)=D_t(u)$
where ''u'' is a bounded function of ''x''₁,...,''x''_''n'', ''t'' defined for ''t'' ≥ 0. From his estimate Nash was able to deduce a continuity estimate for solutions of the elliptic equation
:$D_i(a^(x)D_ju)=0$ by considering the special case when ''u'' does not depend on ''t''.

Notes

References

*.
*. Reprinted in .
*. "''On the analyticity of extremals of multiple integrals''" (English translation of the title) is a short research announcement disclosing the results detailed later in . While, according to the Complete list of De Giorgi's scientific publication (De Giorgi 2006, p. 6), an English translation should be included in , it is unfortunately missing.
*. Translated in English as "''On the differentiability and the analyticity of extremals of regular multiple integrals''" in .
*. Translated in English as "''An example of discontinuous extremals for a variational problem of elliptic type''" in .
*.
*.
*.
*, translated in English as .
*.
*.
*
*.
– Reprinted as .
– Translated to English by Mary Frances Winston Newson as .
– Reprinted as .
– Translated to French by M. L. Laugel (with additions of Hilbert himself) as .
– There exists also an earlier (and shorter) resume of Hilbert's original talk, translated in French and published as .
*.
*.
– Translated in English as .
*.
*.
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*.
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{{Hilbert's problems

#19
Partial differential equations
Calculus of variations