Which mathematical algorithm is NOT commonly used in retail personalization systems for generating product recommendations?

Personalized product recommendations have become ubiquitous in online retail. Behind these seemingly intuitive suggestions are sophisticated mathematical algorithms processing vast amounts of customer data. This poll tests your knowledge about the mathematical foundations that power modern retail personalization systems that help match customers with products they're likely to purchase.

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Markov Chain Monte Carlo (MCMC) methods
Singular Value Decomposition (SVD)
Schur Decomposition Filtering
k-Nearest Neighbors (k-NN)