Liouville's equation

Short description: Equation in differential geometry

For Liouville's equation in dynamical systems, see Liouville's theorem (Hamiltonian).

For Liouville's equation in quantum mechanics, see Von Neumann equation.

For Liouville's equation in Euclidean space, see Liouville–Bratu–Gelfand equation.

In differential geometry, Liouville's equation, named after Joseph Liouville, is the nonlinear partial differential equation satisfied by the conformal factor f of a metric f²(dx² + dy²) on a surface of constant Gaussian curvature K:

[math]\displaystyle{ \Delta_0\log f = -K f^2, }[/math]

where ∆₀ is the flat Laplace operator

[math]\displaystyle{ \Delta_0 = \frac{\partial^2}{\partial x^2} +\frac{\partial^2}{\partial y^2} = 4 \frac{\partial}{\partial z} \frac{\partial}{\partial \bar z}. }[/math]

Liouville's equation appears in the study of isothermal coordinates in differential geometry: the independent variables x,y are the coordinates, while f can be described as the conformal factor with respect to the flat metric. Occasionally it is the square f² that is referred to as the conformal factor, instead of f itself.

Liouville's equation was also taken as an example by David Hilbert in the formulation of his nineteenth problem.

Other common forms of Liouville's equation

By using the change of variables log f ↦ u, another commonly found form of Liouville's equation is obtained:

[math]\displaystyle{ \Delta_0 u = - K e^{2u}. }[/math]

Other two forms of the equation, commonly found in the literature, are obtained by using the slight variant 2 log f ↦ u of the previous change of variables and Wirtinger calculus: [math]\displaystyle{ \Delta_0 u = - 2K e^{u}\quad\Longleftrightarrow\quad \frac{\partial^2 u}{{\partial z}{\partial \bar z}} = - \frac{K}{2} e^{u}. }[/math]

Note that it is exactly in the first one of the preceding two forms that Liouville's equation was cited by David Hilbert in the formulation of his nineteenth problem.

A formulation using the Laplace–Beltrami operator

In a more invariant fashion, the equation can be written in terms of the intrinsic Laplace–Beltrami operator

[math]\displaystyle{ \Delta_{\mathrm{LB}} = \frac{1}{f^2} \Delta_0 }[/math]

as follows:

[math]\displaystyle{ \Delta_{\mathrm{LB}}\log\; f = -K. }[/math]

Properties

Relation to Gauss–Codazzi equations

Liouville's equation is equivalent to the Gauss–Codazzi equations for minimal immersions into the 3-space, when the metric is written in isothermal coordinates [math]\displaystyle{ z }[/math] such that the Hopf differential is [math]\displaystyle{ \mathrm{d}z^2 }[/math].

General solution of the equation

In a simply connected domain Ω, the general solution of Liouville's equation can be found by using Wirtinger calculus. Its form is given by

[math]\displaystyle{ u(z,\bar z) = \ln \left( 4 \frac{ \left|{\mathrm{d} f(z)}/{\mathrm{d} z}\right|^2 }{ ( 1+K \left|f(z)\right|^2)^2 } \right) }[/math]

where f (z) is any meromorphic function such that

• df/dz(z) ≠ 0 for every z ∈ Ω.
• f (z) has at most simple poles in Ω.

Application

Liouville's equation can be used to prove the following classification results for surfaces:

Theorem. A surface in the Euclidean 3-space with metric dl² = g(z,_z)dzd_z, and with constant scalar curvature K is locally isometric to:

1. the sphere if K > 0;
2. the Euclidean plane if K = 0;
3. the Lobachevskian plane if K < 0.

See also

• Liouville field theory, a two-dimensional conformal field theory whose classical equation of motion is a generalization of Liouville's equation

Works cited

• Dubrovin, B. A.; Novikov, S. P.; Fomenko, A. T. (1992), Modern Geometry–Methods and Applications. Part I. The Geometry of Surfaces, Transformation Groups, and Fields, Graduate Studies in Mathematics, 93 (2nd ed.), Berlin–Heidelberg–New York: Springer Verlag, pp. xv+468, ISBN 3-540-97663-9 .
• Henrici, Peter (1993), Applied and Computational Complex Analysis, Wiley Classics Library, 3 (Reprint ed.), New York - Chichester - Brisbane - Toronto - Singapore: John Wiley & Sons, pp. X+637, ISBN 0-471-58986-1, https://books.google.com/books?id=vKZPsjaXuF4C .
• Hilbert, David (1900), "Mathematische Probleme" (in German), Nachrichten von der Königlichen Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-Physikalische Klasse (3): 253–297, http://gdz.sub.uni-goettingen.de/dms/load/img/?PPN=PPN252457811_1900&DMDID=DMDLOG_0037 , translated into English by Mary Frances Winston Newson as Hilbert, David (1902), "Mathematical Problems", Bulletin of the American Mathematical Society 8 (10): 437–479, doi:10.1090/S0002-9904-1902-00923-3, https://www.ams.org/journals/bull/1902-08-10/S0002-9904-1902-00923-3/ .