To cite package ‘expm’ in publications use:

  Maechler M, Dutang C, Goulet V (2024). _expm: Matrix Exponential,
  Log, 'etc'_. R package version 1.0-0,
  <https://r-forge.r-project.org/projects/expm>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {expm: Matrix Exponential, Log, 'etc'},
    author = {Martin Maechler and Christophe Dutang and Vincent Goulet},
    year = {2024},
    note = {R package version 1.0-0},
    url = {https://r-forge.r-project.org/projects/expm},
  }