# Green's identities

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In mathematics, **Green's identities** are a set of three identities in vector calculus. They are named after the mathematician George Green, who discovered Green's theorem.

## Green's first identity

This identity is derived from the divergence theorem applied to the vector field **F** = *ψ*∇*φ*: Let Template:Mvar and Template:Mvar be scalar functions defined on some region *U* ⊂ **R**^{3}, and suppose that Template:Mvar is twice continuously differentiable, and Template:Mvar is once continuously differentiable. Then^{[1]}

where is the Laplace operator, ∂*U* is the boundary of region Template:Mvar, **n** is the outward pointing unit normal of surface element *dS* and Template:Mvar**S** is the oriented surface element. This theorem is a special case of the divergence theorem, and is essentially the higher dimensional equivalent of integration by parts with Template:Mvar and the gradient of Template:Mvar replacing Template:Mvar and Template:Mvar.

Note that Green's first identity above is a special case of the more general identity derived from the divergence theorem by substituting **F** = *ψ***Γ**:

## Green's second identity

If Template:Mvar and Template:Mvar are both twice continuously differentiable on *U* ⊂ **R**^{3}, and Template:Mvar is once continuously differentiable, we can choose **F** = *ψε*∇*φ* − *φε*∇*ψ* and obtain:

For the special case of *ε* = 1 all across *U* ⊂ **R**^{3} then:

In the equation above ∂*φ*/∂**n** is the directional derivative of Template:Mvar in the direction of the outward pointing normal **n** to the surface element *dS*:

In particular, this demonstrates that the Laplacian is self-adjoint in the L2 inner product for functions vanishing on the boundary.

## Green's third identity

Green's third identity derives from the second identity by choosing *φ* = *G*, where the Green's function Template:Mvar is taken to be a fundamental solution of the Laplace operator. This means that:

For example in **R**^{3}, a solution has the form:

Green's third identity states that if Template:Mvar is a function that is twice continuously differentiable on Template:Mvar, then

A simplification arises if Template:Mvar is itself a harmonic function, i.e. a solution to the Laplace equation. Then ∇^{2}*ψ* = 0 and the identity simplifies to:

The second term in the integral above can be eliminated if we choose Template:Mvar to be the Green's function for the boundary of the region Template:Mvar where the problem is posed (Dirichlet boundary condition):

This form is used to construct solutions to Dirichlet boundary condition problems. To find solutions for Neumann boundary condition problems, the Green's function with vanishing normal gradient on the boundary is used instead.

It can be further verified that the above identity also applies when Template:Mvar is a solution to the Helmholtz equation or wave equation and Template:Mvar is the appropriate Green's function. In such a context, this identity is the mathematical expression of the Huygens Principle.

## On manifolds

Green's identities hold on a Riemannian manifold, In this setting, the first two are

where Template:Mvar and Template:Mvar are smooth real-valued functions on Template:Mvar, Template:Mvar is the volume form compatible with the metric, is the induced volume form on the boundary of Template:Mvar, Template:Mvar is oriented unit vector field normal to the boundary, and ∇^{2}*u* = div(grad *u*) is the Laplacian.

## Green's vector identity

Green’s second identity establishes a relationship between second and (the divergence of) first order derivatives of two scalar functions. In differential form

where *p _{m}* and

*q*are two arbitrary twice continuously differentiable scalar fields. This identity is of great importance in physics because continuity equations can thus be established for scalar fields such as mass or energy.

_{m}^{[2]}Although the second Green’s identity is always presented in vector analysis, only a scalar version is found on textbooks. Even in the specialized literature, a vector version is not easily found. In vector diffraction theory, two versions of Green’s second identity are introduced. One variant invokes the divergence of a cross product

^{[3]}

^{[4]}

^{[5]}and states a relationship in terms of the curl-curl of the field

This equation can be written in terms of the Laplacians:

However, the terms

could not be readily written in terms of a divergence. The other approach introduces bi-vectors, this formulation requires a dyadic Green function.^{[6]}^{[7]} The derivation presented here avoids these problems.^{[8]}

Consider that the scalar fields in Green's second identity are the Cartesian components of vector fields, i.e.

Summing up the equation for each component, we obtain

The LHS according to the definition of the dot product may be written in vector form as

The RHS is a bit more awkward to express in terms of vector operators. Due to the distributivity of the divergence operator over addition, the sum of the divergence is equal to the divergence of the sum, i.e.

Recall the vector identity for the gradient of a dot product

which, written out in vector components is given by

This result is similar to what we wish to evince in vector terms 'except' for the minus sign. Since the differential operators in each term act either over one vector (say ’s) or the other (’s), the contribution to each term must be

These results can be rigorously proven to be correct through evaluation of the vector components. Therefore, the RHS can be written in vector form as

Putting together these two results, a result analogous to Green’s theorem for scalar fields is obtained:

The curl of a cross product can be written as

Green’s vector identity can then be rewritten as

Since the divergence of a curl is zero, the third term vanishes and we obtain:

With a similar procedure, the Laplacian of the dot product can be expressed in terms of the Laplacians of the factors

As a corollary, the awkward terms can now be written in terms of a divergence by comparison with the vector Green equation

This result can be verified by expanding the divergence of a scalar times a vector on the RHS.

## See also

## References

- ↑ {{#invoke:citation/CS1|citation |CitationClass=book }}
- ↑ M. Fernández-Guasti. Complementary fields conservation equation derived from the scalar wave equation.
*J. Phys. A: Math. Gen.*, 37:4107–4121, 2004. - ↑ A. E. H. Love. The Integration of the Equations of Propagation of Electric Waves.
*Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character*, 197:pp. 1–45, 1901. - ↑ J. A. Stratton and L. J. Chu. Diffraction Theory of Electromagnetic Waves.
*Phys. Rev.*, 56(1):99–107, Jul 1939. - ↑ N. C. Bruce. Double scatter vector-wave Kirchhoff scattering from perfectly conducting surfaces with infinite slopes.
*Journal of Optics*, 12(8):085701, 2010. - ↑ W. Franz, On the Theory of Diffraction.
*Proceedings of the Physical Society. Section A*, 63(9):925, 1950. - ↑ Chen-To Tai. Kirchhoff theory: Scalar, vector, or dyadic?
*Antennas and Propagation, IEEE Transactions on*, 20(1):114–115, jan 1972. - ↑ M. Fernández-Guasti. Green's second identity for vector fields. ISRN Mathematical Physics, 2012:7, 2012. Article ID: 973968. [1]

## External links

- {{#invoke:citation/CS1|citation

|CitationClass=citation }}

- [2] Green's Identities at Wolfram MathWorld