Which mathematical algorithm is NOT commonly used in retail recommendation systems for product suggestions?

Recommendation systems are crucial in modern retail, helping customers discover products they might like based on their preferences and behaviors. These systems rely heavily on mathematical algorithms and models to make accurate predictions. Test your knowledge about the mathematical foundations of retail recommendation systems that power your shopping experiences online and in-store.

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Markov Chain Monte Carlo (MCMC)
Riemann Zeta Function
Collaborative Filtering with Singular Value Decomposition
Gradient Boosting Decision Trees