Define h : ℂ → ℂ2 via
h(z) = (f(z), g(z)),
where f, g : ℂ → ℂ are analytic functions such that the complex velocity h'(z) = (f'(z), g'(z)) ∈ ℂ2 is never equal to (0,0). Suppose further that h is an embedding of ℂ into ℂ2.
Then the image h(ℂ) is a smooth (C∞) real surface at each of its points h(z), deriving its metric in the standard way as a submanifold of ℂ2 = ℝ4. As such it has a well-defined gaussian curvature K(z) at each point h(z) ∈ ℂ2.
After finding a formula for K(z) in no book or paper that I was able to access, I calculated such a formula as
K(z) = -2 |h''(z) ^ h'(z)|2 / |h'(z)|6,
where h''(z) ^ h'(z) is shorthand for f''(z) g'(z) - g''(z) f'(z).
Question: Is this formula well known? Can anyone give a reference for where it can be found?
(Note: I am not looking for formulas from which this one can be derived.)
Remark: We could have replaced the domain by a nonempty open subset of ℂ and asked only that h be an immersion, but those refinements seemed to obscure the main point.