Harmonic map explained

In the mathematical field of differential geometry, a smooth map between Riemannian manifolds is called harmonic if its coordinate representatives satisfy a certain nonlinear partial differential equation. This partial differential equation for a mapping also arises as the Euler-Lagrange equation of a functional called the Dirichlet energy. As such, the theory of harmonic maps contains both the theory of unit-speed geodesics in Riemannian geometry and the theory of harmonic functions.

Informally, the Dirichlet energy of a mapping from a Riemannian manifold to a Riemannian manifold can be thought of as the total amount that stretches in allocating each of its elements to a point of . For instance, an unstretched rubber band and a smooth stone can both be naturally viewed as Riemannian manifolds. Any way of stretching the rubber band over the stone can be viewed as a mapping between these manifolds, and the total tension involved is represented by the Dirichlet energy. Harmonicity of such a mapping means that, given any hypothetical way of physically deforming the given stretch, the tension (when considered as a function of time) has first derivative equal to zero when the deformation begins.

The theory of harmonic maps was initiated in 1964 by James Eells and Joseph Sampson, who showed that in certain geometric contexts, arbitrary maps could be deformed into harmonic maps. Their work was the inspiration for Richard Hamilton's initial work on the Ricci flow. Harmonic maps and the associated harmonic map heat flow, in and of themselves, are among the most widely studied topics in the field of geometric analysis.

The discovery of the "bubbling" of sequences of harmonic maps, due to Jonathan Sacks and Karen Uhlenbeck, has been particularly influential, as their analysis has been adapted to many other geometric contexts. Notably, Uhlenbeck's parallel discovery of bubbling of Yang–Mills fields is important in Simon Donaldson's work on four-dimensional manifolds, and Mikhael Gromov's later discovery of bubbling of pseudoholomorphic curves is significant in applications to symplectic geometry and quantum cohomology. The techniques used by Richard Schoen and Uhlenbeck to study the regularity theory of harmonic maps have likewise been the inspiration for the development of many analytic methods in geometric analysis.

Geometry of mappings between manifolds

Here the geometry of a smooth mapping between Riemannian manifolds is considered via local coordinates and, equivalently, via linear algebra. Such a mapping defines both a first fundamental form and second fundamental form. The Laplacian (also called tension field) is defined via the second fundamental form, and its vanishing is the condition for the map to be harmonic. The definitions extend without modification to the setting of pseudo-Riemannian manifolds.

Local coordinates

Let be an open subset of and let be an open subset of . For each and between 1 and, let be a smooth real-valued function on, such that for each in, one has that the matrix is symmetric and positive-definite. For each and between 1 and, let be a smooth real-valued function on, such that for each in, one has that the matrix is symmetric and positive-definite. Denote the inverse matrices by and .

For each between 1 and and each between 1 and define the Christoffel symbols and by

\begin{align} \Gamma(g) ^{k& = (1)/(2)} _{ij} \sum ^{m} _{\ell =1} g ^{k\ell} ( (\partial g _{j\ell})/(+ \partial x ^{i}) (\partial g _{i\ell})/(- \partial x ^{j}) (\partial g _{ij})/(\partial x ^{\ell}) ) \\ \Gamma(h) ^{\gamma& = (1)/(2)} _{\alpha\beta} \sum ^{n} _{\delta =1} h ^{\gamma\delta} ( (\partial h _{\beta\delta})/(+ \partial y ^{\alpha}) (\partial h _{\alpha\delta})/(- \partial y ^{\beta}) (\partial h _{\alpha\beta})/(\partial y ^{\delta}) ) \end{align} Given a smooth map from to, its second fundamental form defines for each and between 1 and and for each between 1 and the real-valued function on by\nabla(df) ^{\alpha = (\partial ^{2f} ^{\alpha})/(\partial x ^{i\partial} x ^{j})} _{ij} - \sum ^{k (\partial f ^{\alpha})/(\partial x ^{k})} _{ij} + \sum ^{n (\partial f ^{\beta})/(\partial x ^{i})} _{\gamma =1} (\partial f ^{\gamma})/(\partial x ^{j}) \Gamma(h) ^{\alpha\circ} _{\beta\gamma} f. Its laplacian defines for each between 1 and the real-valued function on by(\Delta f) ^{mg} _{j =1} ^{ij} \nabla(df) ^{\alpha.} _{ij}

Bundle formalism

Let and be Riemannian manifolds. Given a smooth map from to, one can consider its differential as a section of the vector bundle over ; this is to say that for each in, one has a linear map between tangent spaces . The vector bundle has a connection induced from the Levi-Civita connections on and . So one may take the covariant derivative, which is a section of the vector bundle over ; this is to say that for each in, one has a bilinear map of tangent spaces . This section is known as the hessian of .

Using, one may trace the hessian of to arrive at the laplacian of, which is a section of the bundle over ; this says that the laplacian of assigns to each in an element of the tangent space . By the definition of the trace operator, the laplacian may be written as

(\Delta f) _{p=\sum} ^{m ( \nabla(df) )} ^{p(e} _{i,e} _{i)} where is any -orthonormal basis of .

Dirichlet energy and its variation formulas

From the perspective of local coordinates, as given above, the energy density of a mapping is the real-valued function on given by

(1)/(2) \sum ^{n} _{\beta =1} g ^{ij} (\partial f ^{\alpha})/(\partial x ^{i}) (\partial f ^{\beta})/(\partial x ^{j}) (h _{\alpha\beta} \circ f). Alternatively, in the bundle formalism, the Riemannian metrics on and induce a bundle metric on, and so one may define the energy density as the smooth function on . It is also possible to consider the energy density as being given by (half of) the -trace of the first fundamental form. Regardless of the perspective taken, the energy density is a function on which is smooth and nonnegative. If is oriented and is compact, the Dirichlet energy of is defined asE(f) = \int _{M} e(f)d\mu _{g} where is the volume form on induced by . Since any nonnegative measurable function has a well-defined Lebesgue integral, it is not necessary to place the restriction that is compact; however, then the Dirichlet energy could be infinite.

The variation formulas for the Dirichlet energy compute the derivatives of the Dirichlet energy as the mapping is deformed. To this end, consider a one-parameter family of maps with for which there exists a precompact open set of such that for all ; one supposes that the parametrized family is smooth in the sense that the associated map given by is smooth.

• The first variation formula says that

\int _{M} (\partial)/(\partial s) | _{s=0} e(f _{s)d\mu} _{g=-\int} _{M} h\left( (\partial)/(\partial s) | _{s=0} f _{s,\Delta} f\right)d\mu _{g}

There is also a version for manifolds with boundary.

• There is also a second variation formula.

Due to the first variation formula, the Laplacian of can be thought of as the gradient of the Dirichlet energy; correspondingly, a harmonic map is a critical point of the Dirichlet energy. This can be done formally in the language of global analysis and Banach manifolds.

Examples of harmonic maps

Let and be smooth Riemannian manifolds. The notation is used to refer to the standard Riemannian metric on Euclidean space.

• Every totally geodesic map is harmonic; this follows directly from the above definitions. As special cases:
  • For any in, the constant map valued at is harmonic.
  • The identity map is harmonic.
• If is an immersion, then is harmonic if and only if is minimal relative to . As a special case:
  • If is a constant-speed immersion, then is harmonic if and only if solves the geodesic differential equation.

Recall that if is one-dimensional, then minimality of is equivalent to being geodesic, although this does not imply that it is a constant-speed parametrization, and hence does not imply that solves the geodesic differential equation.

• A smooth map is harmonic if and only if each of its component functions are harmonic as maps . This coincides with the notion of harmonicity provided by the Laplace-Beltrami operator.
• Every holomorphic map between Kähler manifolds is harmonic.
• Every harmonic morphism between Riemannian manifolds is harmonic.

Harmonic map heat flow

Well-posedness

Let and be smooth Riemannian manifolds. A harmonic map heat flow on an interval assigns to each in a twice-differentiable map in such a way that, for each in, the map given by is differentiable, and its derivative at a given value of is, as a vector in, equal to . This is usually abbreviated as:

(\partial f)/(\partial t) = \Delta f. Eells and Sampson introduced the harmonic map heat flow and proved the following fundamental properties:

• Regularity. Any harmonic map heat flow is smooth as a map given by .

Now suppose that is a closed manifold and is geodesically complete.

• Existence. Given a continuously differentiable map from to, there exists a positive number and a harmonic map heat flow on the interval such that converges to in the topology as decreases to 0.^[1]
• Uniqueness. If

Notes and References

1. This means that, relative to any local coordinate charts, one has uniform convergence on compact sets of the functions and their first partial derivatives.