N^* P N = [[Fraction(0, 1), Fraction(0, 1)], [Fraction(0, 1), Fraction(4, 1)]]
P - N^* P N = [[Fraction(1, 1), Fraction(0, 1)], [Fraction(0, 1), Fraction(0, 1)]]
kappa(P) = 4; hence sqrt(kappa(P)) = 2
Sharpness: the (2,2) Stein entry forces p22 >= 4 p11,
so lambda_max(P) >= 4 lambda_min(P) for every feasible P.
