# Isothermal coordinates

In mathematics, specifically in differential geometry, isothermal coordinates on a Riemannian manifold are local coordinates where the metric is conformal to the Euclidean metric. This means that in isothermal coordinates, the Riemannian metric locally has the form

${\displaystyle g=\varphi (dx_{1}^{2}+\cdots +dx_{n}^{2}),}$

where ${\displaystyle \varphi }$ is a smooth function. (If the Riemannian manifold is oriented, some authors insist that a coordinate system must agree with that orientation to be isothermal.)

Isothermal coordinates on surfaces were first introduced by Gauss. Korn and Lichtenstein proved that isothermal coordinates exist around any point on a two dimensional Riemannian manifold. On higher-dimensional Riemannian manifolds a necessary and sufficient condition for their local existence is the vanishing of the Weyl tensor and of the Cotton tensor.

## Isothermal coordinates on surfaces

Gauss (1822) proved the existence of isothermal coordinates on an arbitrary surface with a real analytic metric, following results of Lagrange (1779) on surfaces of revolution. Results for Hölder continuous metrics were obtained by Korn (1916) and Lichtenstein (1916). Later accounts were given by Morrey (1938), Ahlfors (1955), Bers (1952) and Chern (1955). A particularly simple account using the Hodge star operator is given in DeTurck & Kazdan (1981).

### Beltrami equation

The existence of isothermal coordinates can be proved[1] by applying known existence theorems for the Beltrami equation, which rely on Lp estimates for singular integral operators of Calderón and Zygmund.[2][3] A simpler approach to the Beltrami equation has been given more recently by the late Adrien Douady.[4]

If the Riemannian metric is given locally as

${\displaystyle ds^{2}=E\,dx^{2}+2F\,dx\,dy+G\,dy^{2},}$

then in the complex coordinate z = x + iy, it takes the form

${\displaystyle ds^{2}=\lambda |\,dz+\mu \,d{\overline {z}}|^{2},}$

where λ and μ are smooth with λ > 0 and |μ| < 1. In fact

${\displaystyle \lambda ={1 \over 4}(E+G+2{\sqrt {EG-F^{2}}}),\,\,\,\mu =(E-G+2iF)/4\lambda .}$

In isothermal coordinates (u, v) the metric should take the form

${\displaystyle ds^{2}=\rho (du^{2}+dv^{2})}$

with ρ > 0 smooth. The complex coordinate w = u + i v satisfies

${\displaystyle \rho \,|dw|^{2}=\rho |w_{z}|^{2}|\,dz+{w_{\overline {z}} \over w_{z}}\,d{\overline {z}}|^{2},}$

so that the coordinates (u, v) will be isothermal if the Beltrami equation

${\displaystyle {\partial w \over \partial {\overline {z}}}=\mu {\partial w \over \partial z}}$

has a diffeomorphic solution. Such a solution has been proved to exist in any neighbourhood where ||μ|| < 1.

### Hodge star operator

New coordinates u and v are isothermal provided that

${\displaystyle \star du=dv,}$

where ${\displaystyle \star }$ is the Hodge star operator defined by the metric.[5]

Let ${\displaystyle \Delta =d^{*}d}$ be the Laplace–Beltrami operator on functions.

Then by standard elliptic theory, u can be chosen to be harmonic near a given point, i.e. Δ u = 0, with du non-vanishing.[5][6]

Indeed, since the problem is local, it is sufficient to describe a solution on the torus T2 endowed with a Riemannian metric. In this case Δ f = g can be solved near 0 with given initial values f(0), df(0).
This can be proved using the L2 Sobolev spaces Hs(T2) for s ≥ 0.[7] These Hilbert spaces can be defined in terms of Δ and the Riemannian structure, but are independent of these structures. It follows that I + Δ gives a linear isomorphism from Hs+2(T2) onto Hs(T2) and that Δ f = g is solvable if and only if g is orthogonal to the constants. On the other hand standard techniques imply an approximation theorem:[8] smooth functions which vanish in a neighbourhood of a point are dense in Hs(T2) for s ≤ 1 (for the method of proof, see below).
In particular density implies that for any s > 0 small there are smooth functions g equal to 0 near 1, orthogonal to the constants in Hs(T2) such that the functions f = ∆−1 g are dense in the subspace of Hs+2(T2) orthogonal to constants. By elliptic regularity, these f are smooth. By the Sobolev embedding theorem Hs+2(T2) lies in C1(T2); density in the Sobolev space implies that f(0), df(0) take all possible values, as claimed.
The approximation theorem above can be proved using the same methods as the corresponding 1-dimensional result: smooth functions which vanish in a neighbourhood of a point are dense in Hs(T) for s ≤ 1/2. For simplicity only this case will be described. It suffices to prove this for the point 1 on the unit circle T. By Cayley transform between the circle and the real line, functions vanishing to infinite order at 1 in C(T) can be identified with S(R), the space of Schwartz functions on R. Smooth functions of compact support are dense in S(R); and hence A the space of smooth functions vanishing in a neighbourhood of 1 in C(T) is dense in the space of smooth functions vanishing with all their derivatives at 1. By the Stone-Weierstrass theorem, A is uniformly dense in C0(T\{1}). Thus if h lies in B, the ideal in C1(T) of functions vanishing with their derivative at 1, h and h' can be uniformly approximated by a function in A. Hence A is dense in B. On the other hand C1(T) is in Hs(T) if s ≤ 1/2. To prove that A is dense in Hs(T), it therefore suffices to show it contains functions an(θ}}) and bn(θ) tending to zero in Sobolev norm with an(0) = 0 vanishing at 1 and ∂θan(0) = 1; and bn(0) = 1 ad ∂θbn(0) = 0. Suitable functions are an(θ) = sin nθ / n and bn(θ) = cn(θ) / cn(0) where cn(θ) = ∑ (1 − n−1)k cos kθ / k log k}}.[9]

By the Poincaré lemma ${\displaystyle \star du=dv}$ has a local solution v exactly when ${\displaystyle d\star du=0}$.

Since

${\displaystyle \star d\star =d^{*},}$

this is equivalent to Δ u = 0, and hence a local solution exists.

Since du is non-zero and the square of the Hodge star operator is −1 on 1-forms, du and dv are necessarily linearly independent, and therefore u and v give local isothermal coordinates.

### Gaussian curvature

In the isothermal coordinates (u, v), the Gaussian curvature takes the simpler form

${\displaystyle K=-{\frac {1}{2}}e^{-\rho }\left({\frac {\partial ^{2}\rho }{\partial u^{2}}}+{\frac {\partial ^{2}\rho }{\partial v^{2}}}\right),}$

where ${\displaystyle \varphi =e^{\rho }}$.

## References

• Ahlfors, Lars V. (1952), Conformality with respect to Riemannian metrics., Ann. Acad. Sci. Fenn. Ser. A. I., 206, pp. 1–22
• Ahlfors, Lars V. (1966), Lectures on quasiconformal mappings, Van Nostrand
• Bers, Lipman (1952), Riemann Surfaces, 1951–1952, New York University, pp. 15–35
• Bers, Lipman; John, Fritz; Schechter, Martin (1979), Partial differential equations, Lectures in Applied Mathematics, 3A, American Mathematical Society, ISBN 0-8218-0049-3
• Chern, Shiing-shen (1955), "An elementary proof of the existence of isothermal parameters on a surface", Proc. Amer. Math. Soc., American Mathematical Society, 6 (5): 771–782, doi:10.2307/2032933, JSTOR 2032933
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