To cite fastmatrix in publications use:

  Osorio, F., Ogueda, A. (2024). Fast computation of some matrices
  useful in statistics. R package version 0.5-772. URL:
  https://faosorios.github.io/fastmatrix/

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {Fast computation of some matrices useful in statistics},
    author = {F. Osorio and A. Ogueda},
    year = {2024},
    note = {R package version 0.5-772},
    url = {https://faosorios.github.io/fastmatrix/},
  }