Lagrange interp exactness: P₂(T_k)=y_k; Σ L_k=1; L_k(T_j)=δ_kj

Sympy-exact symbolic witness of the Lagrange-interpolation-exactness identities for a 3-point polynomial. Setup: T, T_0, T_1, T_2, y_0, y_1, y_2 sp-symbols. Lagrange basis functions L_k(T) = Π_{j≠k}(T-T_j)/(T_k-T_j), and interpolating…