Developed foundational group theory features for the leanprover-community/mathlib4 repository, focusing on formal verification and advanced mathematics using Lean. Delivered the Focal Subgroup Theorem, including the implementation of the focalSubgroup and the transfer homomorphism, and formally proved the relationship between Sylow p-subgroups and commutator subgroups in finite groups. The work emphasized rigorous proof construction and mathematical precision, enhancing the library’s support for downstream formalization and research in group theory. Documentation was updated to improve source citation, reflecting a thorough approach to both code and references. No bug fixes were required, as the focus remained on new feature development.