Harmonic differential: Difference between revisions
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== Explanation == |
== Explanation == |
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Consider the case of real one-forms defined on a two dimensional [[real manifold]]. Moreover, consider real one-forms which are the real parts of [[complex number|complex]] differentials. Let |
Consider the case of real one-forms defined on a two dimensional [[real manifold]]. Moreover, consider real one-forms which are the real parts of [[complex number|complex]] differentials. Let <math>\scriptstyle \omega \, = \, A\,\text{d}x \, + \, B\,\text{d}y</math>, and formally define the '''conjugate''' one-form to be <math>\scriptstyle \omega^* \, = \, A\,\text{d}y \, - \, B\,\text{d}x</math>. |
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== Motivation == |
== Motivation == |
Revision as of 17:21, 9 July 2010
In mathematics, a real differential one-form ω is called a harmonic differential if ω and its conjugate one-form, written as ω*, are both closed.
Explanation
Consider the case of real one-forms defined on a two dimensional real manifold. Moreover, consider real one-forms which are the real parts of complex differentials. Let , and formally define the conjugate one-form to be .
Motivation
There is a clear connection with complex analysis. Let us write a complex number z in terms of its real and imaginary parts, say x and y respectively, i.e. z = x + iy. Since ω + iω* = (A − iB)(dx + i dy), from the point of view of complex analysis, the quotient (ω + iω*)/dz tends to a limit as dz tends to 0. In other words, the definition of ω* was chosen for its connection with the concept of a derivative (analyticity). Another connection with the complex unit is that (ω*)* = −ω (just as i2 = −1).
For a given function ƒ, let us write ω = dƒ, i.e. ω = (∂ƒ/∂x) dx + (∂ƒ/∂y) dy where ∂ denotes the partial derivative. Then (dƒ)* = (∂ƒ/∂x) dy − (∂ƒ/∂y) dx. Now d(dƒ)* is not always zero, indeed d(dƒ)* = Δƒ dx dy, where Δƒ = ∂2ƒ/∂x2 + ∂2ƒ/∂y2.
Cauchy–Riemann equations
As we have seen above: we call the one-form ω harmonic if both ω and ω* are closed. This means that ∂A/∂y = ∂B/∂x (ω is closed) and ∂B/∂y = −∂A/∂x (ω* is closed). These are called the Cauchy–Riemann equations on A − iB. Usually they are expressed in terms of u(x, y) + iv(x, y) as ∂u/∂x = ∂v/∂y and ∂v/∂x = −∂u/∂y.
Notable results
- A harmonic differential (one-form) is precisely the real part of an (analytic) complex differential.[1] To prove this one shows that u + iv satisfies the Cauchy–Riemann equations exactly when u + iv is locally an analytic function of x + iy. Of course an analytic function w(z) = u + iv is the local derivative of something (namely ∫w(z) dz)
- The harmonic differentials ω are (locally) precisely the differentials dƒ of solutions ƒ to Laplace's equation Δƒ = 0.[1]
- If ω is a harmonic differential, so is ω*.[1]