To cite package ‘portfolio.optimization’ in publications use:

  Hochreiter R (2018). _portfolio.optimization: Contemporary Portfolio
  Optimization_. R package version 1.0-0,
  <https://github.com/rhochreiter/portfolio.optimization>.

A BibTeX entry for LaTeX users is

  @Manual{,
    title = {portfolio.optimization: Contemporary Portfolio Optimization},
    author = {Ronald Hochreiter},
    year = {2018},
    note = {R package version 1.0-0},
    url = {https://github.com/rhochreiter/portfolio.optimization},
  }