Evaluate at a particular set of parameters the Gompertz function.
gompertz_fn(x, theta)Numeric vector of the same length of x with the values of the
Gompertz function.
numeric vector at which the Gompertz function is to be evaluated.
numeric vector with the four parameters in the form
c(alpha, delta, eta, phi).
The Gompertz function f(x; theta) is defined here as
g(x; theta) = exp(-exp(-eta * (x - phi)))
f(x; theta) = alpha + delta g(x; theta)
where theta = c(alpha, delta, eta, phi), alpha is the value of the
function when x -> -Inf, delta is the (signed) height of the curve,
eta > 0 is the steepness of the curve or growth rate, and phi is related
with the value of function at x = 0.
When delta < 0 the curve is monotonically decreasing while it is
monotonically increasing for delta > 0.