T_2(x) = 2x² − 1: alternation (−1,0,1)→(1,−1,1); roots at ±√2/2

Sympy-exact minimax witness for the smallest non-trivial Chebyshev case. sp.chebyshevt(2, x) expands to 2x² − 1 identically. Evaluation pins: T_2(−1) = 1, T_2(0) = −1, T_2(1) = 1 — the three equioscillation extrema; T_2(cos(π/4)) = 2 ·…