To cite splines2 in publications use:

Wang, W., Yan, J. (2018). splines2: Regression Spline Functions and Classes. R package version 0.2.8. https://CRAN.R-project.org/package=splines2

Corresponding BibTeX entry:

  @Manual{,
    title = {{splines2}: {R}egression Spline Functions and Classes},
    author = {Wenjie Wang and Jun Yan},
    note = {{R} package version 0.2.8},
    year = {2018},
    url = {https://CRAN.R-project.org/package=splines2},
  }