Splines allow you to fit an almost infinitely flexible number of nonlinear curves, but sometimes you want to place restrictions, such as monotonicity or periodicity on those curves. This article shows you how to do this.
Wenjie Wang, Jun Yan. Shape-Restricted Regression Splines with R Package splines2. Journal of Data Science 2021-08-12, 19(3), 498-517. DOI: 10.6339/21-JDS1020. Available in pdf format.