To cite package ‘GPpenalty’ in publications use:

  Mutoh A (2025). _GPpenalty: Penalized Likelihood in Gaussian
  Processes_. R package version 1.0.1,
  <https://CRAN.R-project.org/package=GPpenalty>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {GPpenalty: Penalized Likelihood in Gaussian Processes},
    author = {Ayumi Mutoh},
    year = {2025},
    note = {R package version 1.0.1},
    url = {https://CRAN.R-project.org/package=GPpenalty},
  }