Evaluate at a particular set of parameters the 4-parameter logistic function.
logistic4_fn(x, theta)Numeric vector of the same length of x with the values of the
logistic function.
numeric vector at which the logistic function is to be evaluated.
numeric vector with the four parameters in the form
c(alpha, delta, eta, phi).
The 4-parameter logistic function f(x; theta) is defined here as
g(x; theta) = 1 / (1 + 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 (also known as the
Hill coefficient), and phi is the value of x at which the curve is equal
to its mid-point.
When delta < 0 the curve is monotonically decreasing while it is
monotonically increasing for delta > 0.
The mid-point f(phi; theta) is equal to alpha + delta / 2 while the value
of the function for x -> Inf is alpha + delta.