To cite the 'Ryacas' package in publications use:

  Andersen MM, Højsgaard S (2019). Ryacas: A computer algebra system in
  R. Journal of Open Source Software, 4(42), 1763, URL
  https://doi.org/10.21105/joss.01763

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{Ryacas: A computer algebra system in R}},
    author = {Mikkel Meyer Andersen and Søren Højsgaard},
    journal = {Journal of Open Source Software},
    year = {2019},
    volume = {4},
    number = {42},
    url = {https://doi.org/10.21105/joss.01763},
  }