A₁ Root System: Weyl Group S₂ Underlying SU(2) and Topological Phases — E8 Intelligence Research

FINDING: A₁ root system is the minimal rank-1 case whose Weyl group is S₂ = ℤ₂; its representation theory (Clebsch–Gordan / 3j-symbols) is the algebraic skeleton underlying SU(2) angular momentum and, via Altland–Zirnbauer classes, topological phase classification. MATH: - A₁ root system: roots {±α}, Cartan matrix [2], Weyl group W(A₁) = S₂ = {1, s_α}, s_α(α) = −α. Rank 1, |W| = 2. - Clebsch–Gordan: Vj₁ ⊗ Vj₂ = ⊕J=|j₁−j₂|j₁+j₂ V_J. For A₁ (SU(2)), multiplicity-free. - Wigner 3j-symbol relates to CG: ⟨j₁m₁j₂m₂|JM⟩ = (−1)j₁−j₂+M √(2J+1) · (j₁ j₂ J; m₁ m₂ −M). - Selection rules (triangular): |j₁−j₂| ≤ J ≤ j₁+j₂; m₁+m₂+M = 0. - Orthogonality: Σm₁m₂ (j₁ j₂ j₃; m₁ m₂ m₃)(j₁ j₂ j₃'; m₁ m₂ m₃') = δj₃j₃'δm₃m₃' Δ(j₁j₂j₃), where Δ is the triangle coefficient. - Symmetry: 3j invariant under even permutations of columns; odd permutations give phase (−1)j₁+j₂+j₃. - AZ classification: 2 complex (A, AIII) + 8 real (AI, BDI, D, DIII, AII, CII, C, CI) classes ↔ symmetric spac Author: Andrew Stewart Caldin, Independent Researcher, UK. Part of the E8 Intelligence Research series. Platform: e8intelligence.com