Which e-commerce platform was founded by a mathematician who named it after a mathematical set theory concept?

The intersection of mathematics and e-commerce has led to groundbreaking innovations in how products are marketed and sold. From complex recommendation algorithms to dynamic pricing models, mathematics underpins much of modern online retail. This trivia question explores a unique instance where a mathematical concept became the foundation for a successful online marketplace.

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Etsy (from the Latin 'et si', meaning 'what if', referencing conditional probability)
Wayfair (originally named CSN Stores after the Cauchy sequence notation in calculus)
Alibaba (from Ali Baba and the mathematical concept of 'Open Sesame' as a binary function)
Quora (from 'quorum' in set theory, representing the minimum number of members needed to make a decision)