To cite sphet in publications use:

  Bivand, R., G. Millo, and G. Piras. 2021. A Review of Software for
  Spatial Econometrics in R. Mathematics 9 (11):1276.
  http://dx.doi.org/10.3390/math9111276.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A Review of Software for Spatial Econometrics in {R}},
    author = {Roger Bivand and Giovanni Millo and Gianfranco Piras},
    journal = {Mathematics},
    year = {2021},
    volume = {9},
    number = {11},
    url = {https://www.mdpi.com/2227-7390/9/11/1276},
    doi = {10.3390/math9111276},
  }

  Gianfranco Piras (2010). sphet: Spatial Models with Heteroskedastic
  Innovations in R. Journal of Statistical Software, 35(1), 1-21. DOI
  10.18637/jss.v035.i01

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{sphet}: Spatial Models with Heteroskedastic Innovations in {R}},
    author = {Gianfranco Piras},
    journal = {Journal of Statistical Software},
    year = {2010},
    volume = {35},
    number = {1},
    pages = {1--21},
    doi = {10.18637/jss.v035.i01},
  }

  Roger Bivand, Gianfranco Piras (2015). Comparing Implementations of
  Estimation Methods for Spatial Econometrics. Journal of Statistical
  Software, 63(18), 1-36. DOI 10.18637/jss.v063.i18

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Comparing Implementations of Estimation Methods for Spatial Econometrics},
    author = {Roger Bivand and Gianfranco Piras},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {63},
    number = {18},
    pages = {1--36},
    doi = {10.18637/jss.v063.i18},
  }

  Gianfranco Piras, Paolo Postiglione (2022). A deeper look at impacts
  in spatial Durbin model with {sphet} Geographical Analysis, 54(3),
  664-684. https://doi.org/10.1111/gean.12318

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A deeper look at impacts in spatial Durbin model with {sphet}},
    author = {Gianfranco Piras and Paolo Postiglione},
    journal = {Geographical Analysis},
    year = {2022},
    volume = {54},
    number = {3},
    pages = {664--684},
    doi = {10.1111/gean.12318},
  }