Non-Rectangular Convolutions and (Sub-)Cadences with Three Elements
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The discrete acyclic convolution computes the 2n-1 sums sum_i+j=k; (i,j) in [0,1,2,...,n-1]^2 (a_i b_j) in O(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 sum_i+j=k; (i,j) in (P cap Z^2) (a_i b_j) in a rectangle P with perimeter p in O(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 O(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.
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