Which mathematical algorithm formed the foundation of Amazon's early recommendation system that dramatically increased their e-commerce sales through 'Customers who bought this also bought...' suggestions?

The intersection of mathematics and retail has revolutionized how businesses understand customer behavior and optimize operations. Recommendation algorithms, predictive analytics, and machine learning models have transformed the retail landscape by leveraging mathematical principles. This trivia question explores a significant mathematical innovation that changed how online retailers approach product recommendations.

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Collaborative Filtering using matrix factorization
Random Forest Classification
Gradient Boosting Machines
Deep Neural Networks with Recurrent Layers