Green's identities

  (Redirected from Green's first identity)

In mathematics, Green's identities are a set of three identities in vector calculus relating the bulk with the boundary of a region on which differential operators act. They are named after the mathematician George Green, who discovered Green's theorem.

Green's first identityEdit

This identity is derived from the divergence theorem applied to the vector field F = ψ ∇φ and using the identity that ∇ ·(φ X ) = ∇φ ·X + φ ∇·X: Let φ and ψ be scalar functions defined on some region URd, and suppose that φ is twice continuously differentiable, and ψ is once continuously differentiable. Then[1]


where ∆ ≡ ∇2 is the Laplace operator, U is the boundary of region U, n is the outward pointing unit normal to the surface element dS and dS = ndS 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 ψ and the gradient of φ replacing u and v.

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


Green's second identityEdit

If φ and ψ are both twice continuously differentiable on UR3, and ε is once continuously differentiable, one may choose F = ψε ∇φφε ∇ψ to obtain


For the special case of ε = 1 all across UR3, then,


In the equation above, φ/∂n is the directional derivative of φ in the direction of the outward pointing surface normal n of the surface element dS,


Explicitly incorporating this definition in the Green's second identity with ε = 1 results in


In particular, this demonstrates that the Laplacian is a self-adjoint operator in the L2 inner product for functions vanishing on the boundary so that the right hand side of the above identity is zero.

Green's third identityEdit

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


For example, in R3, a solution has the form


Green's third identity states that if ψ is a function that is twice continuously differentiable on U, then


A simplification arises if ψ is itself a harmonic function, i.e. a solution to the Laplace equation. Then 2ψ = 0 and the identity simplifies to


The second term in the integral above can be eliminated if G is chosen to be the Green's function that vanishes on the boundary of U (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 ψ is a solution to the Helmholtz equation or wave equation and G is the appropriate Green's function. In such a context, this identity is the mathematical expression of the Huygens principle, and leads to Kirchhoff's diffraction formula and other approximations.

On manifoldsEdit

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


where u and v are smooth real-valued functions on M, dV is the volume form compatible with the metric,   is the induced volume form on the boundary of M, N is the outward oriented unit vector field normal to the boundary, and Δu = div(grad u) is the Laplacian.

Green's vector identityEdit

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


where pm and qm 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.[2]

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,

Theorem for vector fields.

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 to yield

Green's vector identity.

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 alsoEdit


  1. ^ Strauss, Walter. Partial Differential Equations: An Introduction. Wiley.
  2. ^ Guasti, M Fernández (2004-03-17). "Complementary fields conservation equation derived from the scalar wave equation". Journal of Physics A: Mathematical and General. IOP Publishing. 37 (13): 4107–4121. doi:10.1088/0305-4470/37/13/013. ISSN 0305-4470.
  3. ^ Love, Augustus E. H. (1901). "I. 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. The Royal Society. 197 (287–299): 1–45. doi:10.1098/rsta.1901.0013. ISSN 0264-3952.
  4. ^ Stratton, J. A.; Chu, L. J. (1939-07-01). "Diffraction Theory of Electromagnetic Waves". Physical Review. American Physical Society (APS). 56 (1): 99–107. doi:10.1103/physrev.56.99. ISSN 0031-899X.
  5. ^ Bruce, Neil C (2010-07-22). "Double scatter vector-wave Kirchhoff scattering from perfectly conducting surfaces with infinite slopes". Journal of Optics. IOP Publishing. 12 (8): 085701. doi:10.1088/2040-8978/12/8/085701. ISSN 2040-8978.
  6. ^ Franz, W (1950-09-01). "On the Theory of Diffraction". Proceedings of the Physical Society. Section A. IOP Publishing. 63 (9): 925–939. doi:10.1088/0370-1298/63/9/301. ISSN 0370-1298.
  7. ^ "Kirchhoff theory: Scalar, vector, or dyadic?". IEEE Transactions on Antennas and Propagation. Institute of Electrical and Electronics Engineers (IEEE). 20 (1): 114–115. 1972. doi:10.1109/tap.1972.1140146. ISSN 0096-1973.
  8. ^ Fernández-Guasti, M. (2012). "Green's Second Identity for Vector Fields". ISRN Mathematical Physics. Hindawi Limited. 2012: 1–7. doi:10.5402/2012/973968. ISSN 2090-4681.

External linksEdit