2
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"Best" = best performing in terms of run-time for exact algorithm and approximation ratio for an approximation algorithm.

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1
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The best-known algorithm for vertex cover in (general) graphs runs in time $O(1.2114^n)$ by Bourgeois, Escoffier, Paschos and van Rooij. You may read their paper and see whether their algorithm can be improved for cubic graphs.

  • Bourgeois, N., Escoffier, B., Paschos, V. T., & van Rooij, J. M. (2012). Fast algorithms for max independent set. Algorithmica, 62(1-2), 382-415.

Moreover, the following paper proves a hardness result for an ($1+\epsilon$) approximation algorithm for vertex cover in cubic graphs.

  • Alimonti, P., & Kann, V. (1997, March). Hardness of approximating problems on cubic graphs. In Italian Conference on Algorithms and Complexity (pp. 288-298). Springer, Berlin, Heidelberg.

However, a greedy 2-approximation algorithm for vertex cover in general graphs is easy. Just pick an uncovered edge each time and choose both of its ends.

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  • $\begingroup$ The paper by Bourgeois et al. that you mention actually provides better running time bounds for cubic graphs (average degree 3). $\endgroup$ – C Komus Nov 22 '18 at 21:44

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