fn_qpl: Quadratic–plateau–linear function

In flexFitR: Flexible Non-Linear Least Square Model Fitting

Quadratic–plateau–linear function

Description

A piecewise function that models an initial quadratic increase from zero up to a plateau, maintains that plateau for a duration, and then changes linearly after the plateau ends.

Usage

fn_qpl(t, t1, t2, dt, b, k, beta)

Arguments

t	A numeric vector of input values (e.g., time).
t1	The onset time of the response. The function is 0 for all values less than t1.
t2	The time when the quadratic growth phase ends and the plateau begins. Must be greater than t1.
dt	Duration of the plateau. Defines t3 = t2 + dt and must be non-negative.
b	Linear coefficient of the quadratic growth phase.
k	The plateau value (level maintained between t2 and t3).
beta	Slope of the final linear phase after t3 (often negative).

Details

The quadratic phase is parameterized so that the curve reaches exactly k at t2. Let \Delta = t_2 - t_1. The quadratic coefficient c is computed internally as:

c = \frac{k - b\Delta}{\Delta^2}.

f(t; t_1, t_2, dt, b, k, \beta) = \begin{cases} 0 & \text{if } t < t_1 \\ b(t - t_1) + c(t - t_1)^2 & \text{if } t_1 \le t \le t_2 \\ k & \text{if } t_2 < t \le t_3 \\ k + \beta (t - t_3) & \text{if } t > t_3 \end{cases}

where t_3 = t_2 + dt.

The function is continuous at t1, t2, and t3. It is not differentiable at t3 unless beta = 0.

Value

A numeric vector of the same length as t, representing the function values.

Examples

library(flexFitR)
plot_fn(
  fn = "fn_qpl",
  params = c(t1 = 30, t2 = 60, dt = 20, b = 0.01, k = 0.9, beta = -0.01),
  interval = c(0, 100),
  n_points = 2000,
  auc_label_size = 3
)

flexFitR documentation built on June 12, 2026, 1:06 a.m.