Which mathematical algorithm is NOT commonly used as a core technique in retail product recommendation systems?

Recommendation systems are a cornerstone of modern e-commerce, helping customers discover products they might like based on their behavior and preferences. Behind these powerful systems are sophisticated mathematical algorithms that analyze vast amounts of data to make predictions. This poll tests your knowledge about which mathematical concepts power the recommendation engines that shape our shopping experiences.

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Markov Decision Processes
Collaborative Filtering
Matrix Factorization
Cosine Similarity Clustering