To cite truncSP in publications use:

  Karlsson M, Lindmark A (2014). “truncSP: An R Package for Estimation
  of Semi-Parametric Truncated Linear Regression Models.” _Statistics
  in Medicine_, *57*(14), 1-19.
  <https://www.jstatsoft.org/article/view/v057i14>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{truncSP}: An {R} Package for Estimation of Semi-Parametric Truncated Linear Regression Models},
    author = {Maria Karlsson and Anita Lindmark},
    journal = {Statistics in Medicine},
    year = {2014},
    volume = {57},
    number = {14},
    pages = {1--19},
    url = {https://www.jstatsoft.org/article/view/v057i14},
  }