To cite X-13ARIMA-SEATS/seasonal in publications use:

  Sax C, Eddelbuettel D (2018). “Seasonal Adjustment by X-13ARIMA-SEATS
  in R.” _Journal of Statistical Software_, *87*(11), 1-17.
  doi:10.18637/jss.v087.i11 <https://doi.org/10.18637/jss.v087.i11>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Seasonal Adjustment by {X-13ARIMA-SEATS} in {R}},
    author = {Christoph Sax and Dirk Eddelbuettel},
    journal = {Journal of Statistical Software},
    year = {2018},
    volume = {87},
    number = {11},
    pages = {1--17},
    doi = {10.18637/jss.v087.i11},
  }