To cite package flexmix in publications use:

Leisch F (2004). “FlexMix: A General Framework for Finite Mixture Models and Latent Class Regression in R.” Journal of Statistical Software, 11(8), 1–18. doi:10.18637/jss.v011.i08.

Grün B, Leisch F (2007). “Fitting Finite Mixtures of Generalized Linear Regressions in R.” Computational Statistics \& Data Analysis, 51(11), 5247–5252. doi:10.1016/j.csda.2006.08.014.

Grün B, Leisch F (2008). “FlexMix Version 2: Finite Mixtures with Concomitant Variables and Varying and Constant Parameters.” Journal of Statistical Software, 28(4), 1–35. doi:10.18637/jss.v028.i04.

Corresponding BibTeX entries:

  @Article{,
    title = {{FlexMix}: A General Framework for Finite Mixture Models
      and Latent Class Regression in {R}},
    author = {Friedrich Leisch},
    journal = {Journal of Statistical Software},
    year = {2004},
    volume = {11},
    number = {8},
    pages = {1--18},
    doi = {10.18637/jss.v011.i08},
  }

  @Article{,
    title = {Fitting Finite Mixtures of Generalized Linear Regressions
      in {R}},
    author = {Bettina Gr\"un and Friedrich Leisch},
    journal = {Computational Statistics \& Data Analysis},
    year = {2007},
    volume = {51},
    number = {11},
    pages = {5247--5252},
    doi = {10.1016/j.csda.2006.08.014},
  }

  @Article{,
    title = {{FlexMix} Version 2: Finite Mixtures with Concomitant
      Variables and Varying and Constant Parameters},
    author = {Bettina Gr\"un and Friedrich Leisch},
    journal = {Journal of Statistical Software},
    year = {2008},
    volume = {28},
    number = {4},
    pages = {1--35},
    doi = {10.18637/jss.v028.i04},
  }