To cite sparseHessianFD in publications use:

Braun M (2017). “sparseHessianFD: An R Package for Estimating Sparse Hessian Matrices.” Journal of Statistical Software, 82(10), 1–22. doi:10.18637/jss.v082.i10.

Corresponding BibTeX entry:

  @Article{,
    title = {{sparseHessianFD}: An {R} Package for Estimating Sparse
      Hessian Matrices},
    author = {Michael Braun},
    journal = {Journal of Statistical Software},
    year = {2017},
    volume = {82},
    number = {10},
    pages = {1--22},
    doi = {10.18637/jss.v082.i10},
  }