To cite NPCirc in publications use:

  Alonso-Pena M., Oliveira M., Ameijeiras-Alonso J., Crujeiras R.M.,
  Gijbels I., Rodriguez-Casal, A. and Scapini, F. (2025). NPCirc:
  Nonparametric Circular Methods (Version 3.2.1).  URL:
  https://CRAN.R-project.org/package=NPCirc.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {{NPCirc}: An {R} Package for Nonparametric Circular Methods},
    author = {Maria Alonso-Pena and Maria Oliveira and Jose Ameijeiras-Alonso and Rosa M. Crujeiras and Irene Gijbels and Alberto Rodriguez-Casal and Felicita Scapini},
    organization = {R package version 3.1.1},
    note = {Completely revised and updated version of NPCirc with new methods and documentation.},
    url = {https://CRAN.R-project.org/package=NPCirc},
  }

  Oliveira M, Crujeiras R, Rodriguez-Casal A (2014). “NPCirc: An R
  Package for Nonparametric Circular Methods.” _Journal of Statistical
  Software_, *61*(9). doi:10.18637/jss.v061.i09
  <https://doi.org/10.18637/jss.v061.i09>.
  <https://www.jstatsoft.org/v61/i09/>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{NPCirc}: An {R} Package for Nonparametric Circular Methods},
    author = {Maria Oliveira and Rosa M. Crujeiras and Alberto Rodriguez-Casal},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {61},
    number = {9},
    doi = {10.18637/jss.v061.i09},
    url = {https://www.jstatsoft.org/v61/i09/},
  }