@Article{	  EsikB98ai,
  title		= "Nonfinite axiomatizability of the equational theory of
		  shuffle",
  journal	= "Acta Informatica",
  year		= "1998",
  volume	= "35",
  number2	= "6",
  pages		= "505--539",
  doi		= "10.1007/s002360050129",
  author	= "{\'E}sik, Zolt{\'a}n and Bertol, Michael"
}