To cite kdecopula in publications use:

Nagler T (2018). “kdecopula: An R Package for the Kernel Estimation of Bivariate Copula Densities.” Journal of Statistical Software, 84(7), 1–22. doi:10.18637/jss.v084.i07.

Corresponding BibTeX entry:

  @Article{,
    title = {{kdecopula}: An {R} Package for the Kernel Estimation of
      Bivariate Copula Densities},
    author = {Thomas Nagler},
    journal = {Journal of Statistical Software},
    year = {2018},
    volume = {84},
    number = {7},
    pages = {1--22},
    doi = {10.18637/jss.v084.i07},
  }