Bell singlet |Ψ⁻⟩: concurrence 1, ρ_A = 𝟙/2, S(ρ_A) = log 2

Sympy-exact matrix verification of the Bell singlet's maximal entanglement via the Wootters 1998 concurrence C = |⟨Ψ|Ψ̃⟩| with |Ψ̃⟩ = (σ_y ⊗ σ_y) |Ψ*⟩, plus the reduced-density-matrix residual and von Neumann entropy. Step 1 — singlet…