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Harmonic differential

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In mathematics, a real differential one-form ω on a surface is called a harmonic differential if ω and its conjugate one-form, written as ω, are both closed.

Explanation

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Consider the case of real one-forms defined on a two dimensional real manifold. Moreover, consider real one-forms that are the real parts of complex differentials. Let ω = A dx + B dy, and formally define the conjugate one-form to be ω = A dyB dx.

Motivation

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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 + = (AiB)(dx + i dy), from the point of view of complex analysis, the quotient (ω + )/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 f, let us write ω = df, i.e. ω = f/x dx + f/y dy, where ∂ denotes the partial derivative. Then (df) = f/x dyf/y dx. Now d((df)) is not always zero, indeed d((df)) = Δf dx dy, where Δf = 2f/x2 + 2f/y2.

Cauchy–Riemann equations

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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 AiB. 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

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  • A harmonic differential (one-form) is precisely the real part of an (analytic) complex differential.[1]: 172  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 df of solutions f to Laplace's equation Δf = 0.[1]: 172 
  • If ω is a harmonic differential, so is ω.[1]: 172 

See also

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References

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  1. ^ a b c Cohn, Harvey (1967), Conformal Mapping on Riemann Surfaces, McGraw-Hill Book Company