Compute the change in the value of a function f
when the function argument increases by delta.
Usage
increment.fv(f, delta)
Arguments
Value
Another object of class "fv" compatible with X.
Details
This command computes the new function
$$g(x) = f(x+h) - f(x-h)$$
where h = delta/2. The value of \(g(x)\) is
the change in the value of \(f\) over an interval of length
delta centred at \(x\).