@article(Cul78,
 author       = "K. Culik, II",
 year         = "1978",
 title        = "The decidability of {$\upsilon $}-local catenativity and of other properties of {${\rm D}0{\rm L}$} systems",
 journal      = "Information Processing Lett.",
 volume       = "7",
 number       = "1",
 pages        = "33--35",
 doi          = "10.1016/0020-0190(78)90035-2",
)
@article(ER78,
 author       = "A. Ehrenfeucht and G. Rozenberg",
 year         = "1978",
 title        = "Simplifications of homomorphisms",
 journal      = "Inform. and Control",
 volume       = "38",
 number       = "3",
 pages        = "298--309",
 doi          = "10.1016/S0019-9958(78)90095-5",
)
@inproceedings(HHKR10,
 author       = "V. Halava and T. Harju and T. K\"arki and M. Rigo",
 year         = "2010",
 title        = "On the periodicity of morphic words",
 editor       = "Y. Gao and H. Lu and S. Seki and S. Yu",
 booktitle    = "Developments in Language Theory, 14th International Conference, DLT 2010, London, ON, Canada, August 17-20, 2010. Proceedings",
 series       = "Lecture Notes in Comput. Sci.",
 volume       = "6224",
 publisher    = "Springer",
 address      = "Berlin",
 pages        = "209--217",
 doi          = "10.1007/978-3-642-14455-4_20",
)
@article(HarLin86,
 author       = "T. Harju and M. Linna",
 year         = "1986",
 title        = "On the periodicity of morphisms on free monoids",
 journal      = "RAIRO Inform. Th\'eor. Appl.",
 volume       = "20",
 number       = "1",
 pages        = "47--54",
)
@article(Hon08,
 author       = "J. Honkala",
 year         = "2008",
 title        = "Cancellation and periodicity properties of iterated morphisms",
 journal      = "Theoret. Comput. Sci.",
 volume       = "391",
 number       = "1-2",
 pages        = "61--64",
 doi          = "10.1016/j.tcs.2007.10.030",
)
@article(Lin68a,
 author       = "A. Lindenmayer",
 year         = "1968",
 title        = "Mathematical models for cellular interactions in development I. Filaments with one-sided inputs",
 journal      = "J. Theoret. Biol.",
 volume       = "18",
 pages        = "280--299",
 doi          = "10.1016/0022-5193(68)90079-9",
)
@article(Lin68b,
 author       = "A. Lindenmayer",
 year         = "1968",
 title        = "Mathematical models for cellular interactions in development II. Simple and branching filaments with two-sided inputs",
 journal      = "J. Theoret. Biol.",
 volume       = "18",
 pages        = "300--315",
 doi          = "10.1016/0022-5193(68)90080-5",
)
@article(Pan86,
 author       = "J.-J. Pansiot",
 year         = "1986",
 title        = "Decidability of periodicity for infinite words",
 journal      = "RAIRO Inform. Th\'eor. Appl.",
 volume       = "20",
 number       = "1",
 pages        = "43--46",
)
@book(RS86,
 author       = "G. Rozenberg and A. Salomaa",
 year         = "1986",
 title        = "The Book of {${\rm L}$}",
 publisher    = "Springer",
 address      = "Berlin",
)
@article(See88,
 author       = "P. S{\'e}{\'e}bold",
 year         = "1988",
 title        = "An effective solution to the {${\rm D}0{\rm L}$}-periodicity problem in the binary case",
 journal      = "EATCS Bull.",
 volume       = "36",
 pages        = "137--151",
)