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The Bunimovich stadium is a rectangle capped with semicircles. When a=0 (circle), orbits are integrable — angles are preserved and motion is regular. Adding the flat sides (a>0) makes the system fully chaotic (ergodic, mixing). A tiny change in initial angle leads to exponentially diverging trajectories — positive Lyapunov exponent. This classical chaos has quantum signatures: level repulsion in eigenvalue statistics (Wigner-Dyson vs Poisson).