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DOI: 10.4230/LIPIcs.ESA.2020.32
URN: urn:nbn:de:0030-drops-128987
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Charalampopoulos, Panagiotis ; Radoszewski, Jakub ; Rytter, Wojciech ; Waleń, Tomasz ; Zuba, Wiktor

The Number of Repetitions in 2D-Strings

LIPIcs-ESA-2020-32.pdf (0.6 MB)


The notions of periodicity and repetitions in strings, and hence these of runs and squares, naturally extend to two-dimensional strings. We consider two types of repetitions in 2D-strings: 2D-runs and quartics (quartics are a 2D-version of squares in standard strings). Amir et al. introduced 2D-runs, showed that there are ?(n³) of them in an n × n 2D-string and presented a simple construction giving a lower bound of Ω(n²) for their number (Theoretical Computer Science, 2020). We make a significant step towards closing the gap between these bounds by showing that the number of 2D-runs in an n × n 2D-string is ?(n² log² n). In particular, our bound implies that the ?(n²log n + output) run-time of the algorithm of Amir et al. for computing 2D-runs is also ?(n² log² n). We expect this result to allow for exploiting 2D-runs algorithmically in the area of 2D pattern matching.
A quartic is a 2D-string composed of 2 × 2 identical blocks (2D-strings) that was introduced by Apostolico and Brimkov (Theoretical Computer Science, 2000), where by quartics they meant only primitively rooted quartics, i.e. built of a primitive block. Here our notion of quartics is more general and analogous to that of squares in 1D-strings. Apostolico and Brimkov showed that there are ?(n² log² n) occurrences of primitively rooted quartics in an n × n 2D-string and that this bound is attainable. Consequently the number of distinct primitively rooted quartics is ?(n² log² n). The straightforward bound for the maximal number of distinct general quartics is ?(n⁴). Here, we prove that the number of distinct general quartics is also ?(n² log² n). This extends the rich combinatorial study of the number of distinct squares in a 1D-string, that was initiated by Fraenkel and Simpson (Journal of Combinatorial Theory, Series A, 1998), to two dimensions.
Finally, we show some algorithmic applications of 2D-runs. Specifically, we present algorithms for computing all occurrences of primitively rooted quartics and counting all general distinct quartics in ?(n² log² n) time, which is quasi-linear with respect to the size of the input. The former algorithm is optimal due to the lower bound of Apostolico and Brimkov. The latter can be seen as a continuation of works on enumeration of distinct squares in 1D-strings using runs (Crochemore et al., Theoretical Computer Science, 2014). However, the methods used in 2D are different because of different properties of 2D-runs and quartics.

BibTeX - Entry

  author =	{Panagiotis Charalampopoulos and Jakub Radoszewski and Wojciech Rytter and Tomasz Waleń and Wiktor Zuba},
  title =	{{The Number of Repetitions in 2D-Strings}},
  booktitle =	{28th Annual European Symposium on Algorithms (ESA 2020)},
  pages =	{32:1--32:18},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-162-7},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{173},
  editor =	{Fabrizio Grandoni and Grzegorz Herman and Peter Sanders},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{},
  URN =		{urn:nbn:de:0030-drops-128987},
  doi =		{10.4230/LIPIcs.ESA.2020.32},
  annote =	{Keywords: 2D-run, quartic, run, square}

Keywords: 2D-run, quartic, run, square
Collection: 28th Annual European Symposium on Algorithms (ESA 2020)
Issue Date: 2020
Date of publication: 26.08.2020

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