A rule of thumb for riffle shuffling

@article{Assaf2009ARO,
  title={A rule of thumb for riffle shuffling},
  author={Sami H. Assaf and Persi Diaconis and Kannan Soundararajan},
  journal={Annals of Applied Probability},
  year={2009},
  volume={21},
  pages={843-875}
}
We study how many riffle shuffles are required to mix n cards if only certain features of the deck are of interest, e.g. suits disregarded or only the colors of interest. For these features, the number of shuffles drops from 3/2 log_2(n) to log_2(n). We derive closed formulae and an asymptotic `rule of thumb' formula which is remarkably accurate. 

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