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Cyclic vector

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2020 Mathematics Subject Classification: Primary: 15A Secondary: 47A1693B [MSN][ZBL]

Let be an endomorphism of a finite-dimensional vector space V. A cyclic vector for A is a vector v such that v,Av,\dots,A^{n-1}v form a basis for V, i.e. such that the pair (A,v) is completely reachable (see also Pole assignment problem; Majorization ordering; System of subvarieties; Frobenius matrix).

A vector v in an (infinite-dimensional) Banach space or Hilbert space with an operator A on it is said to be cyclic if the linear combinations of the vectors A^iv, i=0,1,\dots, form a dense subspace, [a1].

More generally, let \mathcal A be a subalgebra of \mathcal B(H), the algebra of bounded operators on a Hilbert space H. Then v\in H is cyclic if \mathcal Av is dense in H, [a2], [a5].

If \phi is a unitary representation of a (locally compact) group G in H, then v\in H is called cyclic if the linear combinations of the \phi(g)v, g\in G, form a dense set, [a3], [a4]. For the connection between positive-definite functions on G and the cyclic representations (i.e., representations that admit a cyclic vector), see Positive-definite function on a group. An irreducible representation is cyclic with respect to every non-zero vector.

References

[a1] M. Reed, B. Simon, "Methods of mathematical physics: Functional analysis" , 1 , Acad. Press (1972) pp. 226ff
[a2] R.V. Kadison, J.R. Ringrose, "Fundamentals of the theory of operator algebras" , 1 , Acad. Press (1983) pp. 276
[a3] S.A. Gaal, "Linear analysis and representation theory" , Springer (1973) pp. 156
[a4] A.A. Kirillov, "Elements of the theory of representations" , Springer (1976) pp. 53 (In Russian)
[a5] M.A. Naimark, "Normed rings" , Noordhoff (1964) pp. 239 (In Russian) Zbl 0137.31703
How to Cite This Entry:
Cyclic vector. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Cyclic_vector&oldid=52802
This article was adapted from an original article by M. Hazewinkel (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article