Which mathematical concept did Amazon implement in 2003 that revolutionized their product recommendation system and increased cross-selling by over 35%?

In the world of retail, mathematics plays a crucial role in optimizing customer experiences. Advanced algorithms and mathematical models power recommendation systems that drive personalized shopping experiences and boost customer loyalty. This trivia question explores how a major retailer leveraged mathematical principles to revolutionize their recommendation engine and dramatically improve customer engagement metrics.

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Collaborative filtering using matrix factorization
Bayesian probability networks
Neural network deep learning algorithms
Markov decision processes