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The On-Line Encyclopedia of Integer Sequences (OEIS) is a web-accessible database cataloging interesting integer sequences and associated theorems. With more than 390,000 sequences and 12,000 citations, the OEIS is one of the most robust and highly cited resources in all of theoretical mathematics. The Sequencelib project provides an open-source computational platform to formalize the mathematics contained within the OEIS using the Lean programming language. With contributions made through a combination of hand-written formalizations, metaprogramming, and AI, Sequencelib currently contains formalizations for more than 25,000 sequences and over 1.6 million theorems about their values.