The function to differentiate.
The point at which to take the derivative.
A characteristic scale for f.
An approximation to the derivative of f at the point x. The relative error in the result is at best on the order of (real.epsilon)^(2/3), roughly three orders of magnitude more accurate than diff2().
Calculate the derivative of a function using a three-point formula, a.k.a. a central difference formula.
The function is evaluated twice, at points just below and just above x.