# A General Purpose Algorithm for Counting Simple Cycles and Simple Paths of Any Length

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## Abstract

We describe a general purpose algorithm for counting simple cycles and simple paths of any length $$\ell$$ on a (weighted di)graph on N vertices and M edges, achieving an asymptotic running time of $$O\left( N+M+\big (\ell ^\omega +\ell \Delta \big ) |S_\ell |\right)$$. In this expression, $$|S_\ell |$$ is the number of (weakly) connected induced subgraphs of G on at most $$\ell$$ vertices, $$\Delta$$ is the maximum degree of any vertex and $$\omega$$ is the exponent of matrix multiplication. We compare the algorithm complexity both theoretically and experimentally with most of the existing algorithms for the same task. These comparisons show that the algorithm described here is the best general purpose algorithm for the class of graphs where $$(\ell ^{\omega -1}\Delta ^{-1}+1) |S_\ell |\le |\text {Cycle}_\ell |$$, with $$|\text {Cycle}_\ell |$$ the total number of simple cycles of length at most $$\ell$$, including backtracks and self-loops. On Erdős–Rényi random graphs, we find empirically that this happens when the edge probability is larger than circa 4 / N. In addition, we show that some real-world networks also belong to this class. Finally, the algorithm permits the enumeration of simple cycles and simple paths on networks where vertices are labeled from an alphabet on n letters with an asymptotic running time of $$O\left( N+M+\big (n^\ell \ell ^\omega +\ell \Delta \big ) |S_\ell |\right)$$. A Matlab implementation of the algorithm proposed here is available for download.

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## Notes

1. There is one exception to this observation: by extending an approach of Merris to count Hamiltonian cycles [32], we show in Sect. 4 that all simple cycles can be counted with an asymptotic running time scaling as $$\ell \,t_{\text {imm}}(\ell )|S_\ell |$$, where $$t_{\text {imm}}(\ell )$$ is exponential in $$\ell$$. Hence, this extension is still not competitive with the algorithm presented here.

2. Note in this context, backtracks, that is bidirected edges, count as simple cycles. Furthermore the orientation of the cycles counts as well but not their starting point. Thus, for example, the complete graph on three vertices with no self-loops, $$K_3$$, has two simple cycles of length 3 and three of length 2.

3. In addition, we have empirically observed that the running time of the algorithm proposed here scales with an effective parameter $$\Delta _\text {eff}\ll \Delta$$. What determines $$\Delta _\text {eff}$$ remains unclear.

4. Even if a reduction to connected induced subgraphs can be devised for this method, which would yield a $$O(t(S_\ell ) + t_{\text {imm}}(\ell )|S_\ell |)$$ asymptotic running time, it would only marginally improve upon Merris’ approach and would still be worse than that of the algorithm presented here.

5. That is, beyond the fact that AYZ is limited to $$\ell =7$$ on undirected graphs.

6. Variants for counting simple paths and for counting simple paths and simple cycles simultaneously are also available at https://uk.mathworks.com/matlabcentral/fileexchange/63850 and https://uk.mathworks.com/matlabcentral/fileexchange/63849, respectively.

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## Funding

P.-L. Giscard acknowledges financial support from the Royal Commission for the Exhibition of 1851. N. Kriege is supported by the German Science Foundation (DFG) within the Collaborative Research Center SFB 876 “Providing Information by Resource-Constrained Data Analysis”, project A6 “Resource-efficient Graph Mining” .

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Correspondence to Pierre-Louis Giscard.

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Giscard, PL., Kriege, N. & Wilson, R.C. A General Purpose Algorithm for Counting Simple Cycles and Simple Paths of Any Length. Algorithmica 81, 2716–2737 (2019). https://doi.org/10.1007/s00453-019-00552-1

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• DOI: https://doi.org/10.1007/s00453-019-00552-1