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# Non-Rectangular Convolutions and (Sub-)Cadences with Three Elements

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LIPIcs.STACS.2020.30.pdf
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## Acknowledgements

The first author discovered an error in the algorithm for determining the existence of 3-cadences in "String cadences" of Amir et al., which led to false positives. After that, the first author reported this error to Travis Gagie, one of the authors of "String Cadences". Travis Gagie explained this error to the second author during the 30th Annual Symposium on Combinatorial Pattern Matching (CPM 2019) in Pisa. He also claimed that it should be possible to determine the existence of 3-cadences in sub-quadratic time. Juliusz Straszyński showed during the same conference that 3-sub-cadences beginning and ending in given intervals can efficiently be detected by convolution. Amihood Amir noted later in an email that we can also efficiently count these sub-cadences, which allows "subtractive" methods as used for arbitrary triangles.

## Cite As

Mitsuru Funakoshi and Julian Pape-Lange. Non-Rectangular Convolutions and (Sub-)Cadences with Three Elements. In 37th International Symposium on Theoretical Aspects of Computer Science (STACS 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 154, pp. 30:1-30:16, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)
https://doi.org/10.4230/LIPIcs.STACS.2020.30

## Abstract

The discrete acyclic convolution computes the 2n+1 sums ∑_{i+j=k|(i,j)∈[0,1,2,… ,n]²} a_i b_j in ?(n log n) time. By using suitable offsets and setting some of the variables to zero, this method provides a tool to calculate all non-zero sums ∑_{i+j=k|(i,j)∈ P∩ℤ²} a_i b_j in a rectangle P with perimeter p in ?(p log p) time. This paper extends this geometric interpretation in order to allow arbitrary convex polygons P with k vertices and perimeter p. Also, this extended algorithm only needs ?(k + p(log p)² log k) time. Additionally, this paper presents fast algorithms for counting sub-cadences and cadences with 3 elements using this extended method.

## Subject Classification

##### ACM Subject Classification
• Mathematics of computing → Combinatorial algorithms
• Mathematics of computing → Combinatorics on words
• Theory of computation → Computational geometry
• Computing methodologies → Number theory algorithms
##### Keywords
• discrete acyclic convolutions
• geometric algorithms
• number theoretic transforms

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## References

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