Which mathematical concept forms the foundation of collaborative filtering algorithms used in retail personalization?

Behind every personalized product recommendation you see while shopping online lies complex mathematical algorithms. These systems analyze your browsing history, past purchases, demographic information, and even the time you spend looking at certain products. But how much do you know about the mathematical foundations that power these retail personalization systems? Test your knowledge about the algorithms that determine what products you see when shopping online!

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Matrix factorization
Bayesian networks
Fourier transforms
Differential equations