# Shift dynamical system

A dynamical system (or, in a different notation, ) on a space of continuous functions ( is a metric space) equipped with the compact-open topology (that is, the topology of uniform convergence on segments), defined by

where is the shift operator by , that is,

Thus, the trajectory of a point in a shift dynamical system is the set of all shifts of , that is, of all functions of the form for . The closure of the trajectory is the set of all functions of the form

where the limit is uniform on each segment. A shift dynamical system is equipped with normalized invariant measures (cf. Invariant measure); these exist by the Bogolyubov–Krylov theorem (Bogolyubov–Krylov invariant measures are concentrated on compact sets).

A shift dynamical system is used in the theory of dynamical systems mainly to construct examples (here is usually taken to be ; Markov's example of a non-strictly ergodic system on a compact set each trajectory of which is everywhere dense, and others), and also in the theory of non-autonomous systems of ordinary differential equations, where is usually taken to be or a space of mappings (in the theory of linear homogeneous non-autonomous systems it is usual to take ).

See also Singular exponents; Central exponents.

#### Comments

A shift dynamical system as defined above is often called a Bebutov system; cf. [a3]. The Bebutov–Kakutani theorem states that a dynamical system on a compact metric space is isomorphic to a subsystem of the Bebutov system with if and only if the set of its invariant points is homeomorphic to a subset of (cf. [a5], and, for a generalization, [a4]).

Markov's example, mentioned above, can be found in [a7], Chapt. VI, 9.35. For the use of Bebutov systems for non-autonomous systems of ordinary differential equations, see [a8].

Usually, by a shift dynamical system one understands a discrete-time system (a cascade) of the form ; here denotes a finite non-empty set, is the space of all two-sided infinite sequences with elements in , endowed with the usual product topology (this is just with its compact-open topology when is considered with its discrete topology), and is the shift operator by 1, that is, for .

These (discrete) shift systems play an important role in ergodic theory and topological dynamics. For example, a Bernoulli system is a shift system endowed with the product measure on defined by a probability measure on (cf. Bernoulli automorphism). The discrete shift systems and their subsystems (subshifts) are not only used for the construction of special examples (for an important method — substitution — cf. [a6]), they are also important for the study of the behaviour of a large class of cascades by "coding" their trajectories by means of elements of for a suitable set (cf. Symbolic dynamics). More recently, it turned out that the methods used for the classification of so-called subshifts of finite type (see [a2]) are useful for information processing; see [a1].

#### References

[a1] | R.L. Adler, D. Coppersmith, M. Hassner, "Algorithms for sliding block codes" IEEE Trans. Inform. Theory , 29 (1983) pp. 5–22 |

[a2] | R.L. Adler, B. Marcus, "Topological entropy and equivalence of dynamical systems" , Amer. Math. Soc. (1979) |

[a3] | H. Furstenberg, "Recurrence in ergodic theory and combinatorial number theory" , Princeton Univ. Press (1981) |

[a4] | O. Hajek, "Representations of dynamical systems" Funkcial. Ekvac. , 114 (1971) pp. 25–34 |

[a5] | S. Kakutani, "A proof of Bebutov's theorem" J. Differential Equations , 4 (1968) pp. 194–201 |

[a6] | J.C. Martin, "Substitution minimal flows" Amer. J. Math. , 93 (1971) pp. 503–526 |

[a7] | V.V. Nemytskii, V.V. Stepanov, "Qualitative theory of differential equations" , Princeton Univ. Press (1960) (Translated from Russian) |

[a8] | G.R. Sell, "Topological dynamics and ordinary differential equations" , v. Nostrand-Reinhold (1971) |

**How to Cite This Entry:**

Shift dynamical system.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Shift_dynamical_system&oldid=17867