Vasili Nikitin contributed to the leanprover-community/mathlib4 repository by formalizing a key geometric result: the diameter of a Euclidean ball is twice its radius. Using Lean and formal verification techniques, Vasili developed supporting lemmas for spheres and closed balls, and refactored existing proofs in the Analysis/InnerProductSpace/EuclideanDist module to improve clarity and maintainability. The work included restructuring proof strategies and simplifying variable declarations, which enhanced code readability and future extensibility. Collaborating closely with other contributors, Vasili’s efforts provided reusable geometric lemmas that strengthen the foundation for downstream proofs, demonstrating depth in mathematics and proof assistant skills within the mathlib4 ecosystem.