4 research outputs found
Two-Dimensional Maximal Repetitions
Maximal repetitions or runs in strings have a wide array of applications and thus have been extensively studied. In this paper, we extend this notion to 2-dimensions, precisely defining a maximal 2D repetition. We provide initial bounds on the number of maximal 2D repetitions that can occur in a matrix. The main contribution of this paper is the presentation of the first algorithm for locating all maximal 2D repetitions in a matrix. The algorithm is efficient and straightforward, with runtime O(n^2 log n log log n+ rho log n), where n^2 is the size of the input, and rho is the number of 2D repetitions in the output
The Number of Repetitions in 2D-Strings
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 of them in an 2D-string and
presented a simple construction giving a lower bound of for their
number (TCS 2020). We make a significant step towards closing the gap between
these bounds by showing that the number of 2D-runs in an 2D-string
is . In particular, our bound implies that the run-time of the algorithm of Amir et al. for computing
2D-runs is also . 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 identical blocks
(2D-strings) that was introduced by Apostolico and Brimkov (TCS 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 occurrences of primitively rooted quartics in an
2D-string and that this bound is attainable. Consequently the number of
distinct primitively rooted quartics is . Here, we prove that
the number of distinct general quartics is also . This extends
the rich combinatorial study of the number of distinct squares in a 1D-string,
that was initiated by Fraenkel and Simpson (J. Comb. Theory A 1998), to two
dimensions.
Finally, we show some algorithmic applications of 2D-runs. (Abstract
shortened due to arXiv requirements.)Comment: To appear in the ESA 2020 proceeding