An internal model principle is necessary and sufficient for linear output synchronization. Wieland, P., Sepulchre, R., & Allgöwer, F. Automatica, 47(5):1068-1074, 2011.
bibtex   
@ARTICLE{ist:wieland11a,
  author = {Wieland, P. and Sepulchre, R. and Allg{\"o}wer, F.},
  title = {An internal model principle is necessary and sufficient for linear
	output synchronization},
  journal = {Automatica},
  year = {2011},
  volume = {47},
  pages = {1068-1074},
  number = {5},
  pubtype = {journal}
}

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