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.
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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 ω = A dx + B dy, and formally define the conjugate one-form to be ω* = A dy − B dx.
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 f, let us write ω = df, i.e. ω = (∂f/∂x) dx + (∂f/∂y) dy where ∂ denotes the partial derivative. Then (df)* = (∂f/∂x) dy − (∂f/∂y) dx. Now d(df)* is not always zero, indeed d(df)* = Δf dx dy, where Δf = ∂2f/∂x2 + ∂2f/∂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.