To cite splines2 in publications use:

Wang W, Yan J (2021). splines2: Regression Spline Functions and Classes. R package version 0.4.1, https://CRAN.R-project.org/package=splines2.

Corresponding BibTeX entry:

  @Manual{splines2-package,
    title = {{splines2}: {R}egression Spline Functions and Classes},
    author = {Wenjie Wang and Jun Yan},
    year = {2021},
    url = {https://CRAN.R-project.org/package=splines2},
    note = {{R} package version 0.4.1},
  }