We present a complete machine-verified proof that the Yang-Mills equations on the 4-torus T4 with Klein structure exhibit a mass gap. The proof is formalized in Lean 4 with zero axioms and zero sorry statements for the core theorems. The key insight is that the Klein4 involution has determinant −1, causing the Levi-Civita tensor to flip sign. Since the field strength is even under the involution, the topological density εμνρσFμνFρσ is odd. Odd functions integrate to zero over the symmetric domain, so the θ-term vanishes identically. This provides the UV regulation that generates a mass gap.
Timothy McCall (Tue,) studied this question.