Isothermal coordinates

Coordinates of a two-dimensional Riemannian space in which the square of the line element has the form:

$$ ds ^ {2} = \lambda ( \xi , \eta ) ( d \xi ^ {2} + d \eta ^ {2} ). $$

Isothermal coordinates specify a conformal mapping of the two-dimensional Riemannian manifold into the Euclidean plane. Isothermal coordinates can always be introduced in a compact domain of a regular two-dimensional manifold. The Gaussian curvature can be calculated in isothermal coordinates by the formula:

$$ k = - \frac{\Delta \mathop{\rm ln} \lambda } \lambda , $$

where $ \Delta $ is the Laplace operator.

Isothermal coordinates are also considered in two-dimensional pseudo-Riemannian spaces; the square of the line element then has the form:

$$ ds ^ {2} = \psi ( \xi , \eta ) ( d \xi ^ {2} - d \eta ^ {2} ). $$

Here, frequent use is made of coordinates $ \mu , \nu $ which are naturally connected with isothermal coordinates and in which the square of the line element has the form:

$$ ds ^ {2} = \lambda ( \mu , \nu ) d \mu d \nu . $$

In this case the lines $ \mu = \textrm{ const } $ and $ \nu = \textrm{ const } $ are isotropic geodesics and the coordinate system $ \mu , \nu $ is called isotropic. Isotropic coordinates are extensively used in general relativity theory.

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