In physics and mathematics, the **Hadamard dynamical system** or **Hadamard's billiard** is a chaotic dynamical system, a type of dynamical billiards. Introduced by Jacques Hadamard in 1898, it is the first dynamical system to be proven chaotic.

The system considers the motion of a free (frictionless) particle on a surface of constant negative curvature, the simplest compact Riemann surface, which is the surface of genus two: a donut with two holes. Hadamard was able to show that every particle trajectory moves away from every other: that all trajectories have a positive Lyapunov exponent.

Frank Steiner argues that Hadamard's study should be considered to be the first-ever examination of a chaotic dynamical system, and that Hadamard should be considered the first discoverer of chaos. He points out that the study was widely disseminated, and considers the impact of the ideas on the thinking of Albert Einstein and Ernst Mach.

The system is particularly important in that in 1963, Yakov Sinai, in studying Sinai's billiards as a model of the classical ensemble of a Boltzmann–Gibbs gas, was able to show that the motion of the atoms in the gas follow the trajectories in the Hadamard dynamical system.

The motion studied is that of a free particle sliding frictionlessly on the surface, namely, one having the Hamiltonian

H
(
p
,
q
)
=
1
2
m
p
i
p
j
g
i
j
(
q
)
where *m* is the mass of the particle,
q
i
,
i
=
1
,
2
are the coordinates on the manifold,
p
i
are the conjugate momenta:

p
i
=
m
g
i
j
d
q
j
d
t

and

d
s
2
=
g
i
j
(
q
)
d
q
i
d
q
j
is the metric tensor on the manifold. Because this is the free-particle Hamiltonian, the solution to the Hamilton–Jacobi equations of motion are simply given by the geodesics on the manifold.

Hadamard was able to show that all geodesics are unstable, in that they all diverge exponentially from one another, as
e
λ
t
with positive Lyapunov exponent

λ
=
2
E
m
R
2
with *E* the energy of a trajectory, and
K
=
−
1
/
R
2
being the constant negative curvature of the surface.