To cite package ‘SOP’ in publications use:

  Rodriguez-Alvarez M, Oviedo de la Fuente M (2023). _SOP: Generalised
  Additive P-Spline Regression Models Estimation_. R package version
  1.0-1, <https://CRAN.R-project.org/package=SOP>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {SOP: Generalised Additive P-Spline Regression Models Estimation},
    author = {Maria Xose Rodriguez-Alvarez and Manuel {Oviedo de la Fuente}},
    year = {2023},
    note = {R package version 1.0-1},
    url = {https://CRAN.R-project.org/package=SOP},
  }