Which mathematical approach was NOT part of Amazon's original product recommendation system that helped revolutionize online retail?

Recommendation systems power modern retail experiences, but they're built on sophisticated mathematical foundations. This trivia explores how mathematics drives product recommendation algorithms that help retailers increase sales and enhance customer experiences. Understanding these systems requires knowledge of both retail strategy and mathematical concepts that transform raw data into personalized shopping experiences.

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Collaborative filtering using matrix factorization
Neural networks with backpropagation
Item-to-item similarity calculations
Conditional probability models