To cite package ‘optextras’ in publications use:

Nash JC (2019). optextras: Tools to Support Optimization Possibly with Bounds and Masks. R package version 2019-12.4, https://CRAN.R-project.org/package=optextras.

Corresponding BibTeX entry:

  @Manual{,
    title = {optextras: Tools to Support Optimization Possibly with
      Bounds and Masks},
    author = {John C Nash},
    year = {2019},
    note = {R package version 2019-12.4},
    url = {https://CRAN.R-project.org/package=optextras},
  }