Approximation in the mean by polynomials on non-Carathéodory domains

Brennan, James E.

doi:10.1007/bf02386037

Cited by 40 publications

(19 citation statements)

References 35 publications

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“…It is known that for a suitable choice of K (but not for all choices), P will be dense in £. (See the 1953 survey article [23] by Mergelyan; for more recent results see, for example, [6].) Thus £ satisfies the first six axioms.…”

Section: |[K -mentioning

confidence: 92%

“…If/ e E, then zf £ E. 6. To each point weSG (the boundary of G) there corresponds a function/in E that has a singularity at w.…”

Section: Cyclic Vectors In the Dirichlet Space By Leon Brown And Allementioning

confidence: 99%

“…We shall show that these functions converge in norm to g2 as t -» 1. Since (g2/f,)f -g2 vanishes at the origin, it will be sufficient, in view of (6), to consider the Bergman norm of the derivative. Also since g2 converges in norm to g2, we may replace g by gt.…”

Section: Corollary M(e) C £ N Hx(g)mentioning

confidence: 99%

“…Since pnfg -»/ pointwise in A, it suffices to show that ||/>"/g||D < const (see Proposition 5 and the remark following the proof). By (6) it is sufficient to show that \\(p"fg)'\\B < const. We have \\((Png)f)'\\B*i\\Png\\Jf'\\B+\\f\L\\(p"gy\\B, which is bounded since ||(p"g)'||B < ||/>"g||D.…”

Section: Corollarymentioning

confidence: 99%

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Cyclic Vectors in the Dirichlet Space

Brown¹,

Shields²

1984

Transactions of the American Mathematical Society

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Abstract. We study the Hubert space of analytic functions with finite Dirichlet integral in the open unit disc. We try to identify the functions whose polynomial multiples are dense in this space. Theorems 1 and 2 confirm a special case of the following conjecture: if \f(z)\ > \g(z)\ at all points and if g is cyclic, then/is cyclic. Theorems 3-5 give a sufficient condition (/ is an outer function with some smoothness and the boundary zero set is at most countable) and a necessary condition (the radial limit can vanish only for a set of logarithmic capacity zero) for a function / to be cyclic.Introduction. In this paper we shall study the (Hubert) space of analytic functions in the open unit disc A in the complex plane that have a finite Dirichlet integral: // l/'l2 dx dy < oo. Our goal is to identify, as far as possible, the "cyclic vectors" in this space, that is, those functions/such that the polynomial multiples of fare dense in the space. The corresponding problem for the Hardy space H2 was solved by Beurling [5] in 1949: the cychc vectors are precisely the outer functions. The present paper is divided into four sections; it contains 20 propositions, 5 theorems and 19 unsolved problems (stated as Questions in the text).The first section deals with cyclic vectors and multiplication operators in a general Banach space of analytic functions (in a bounded region of the complex plane). The theory is illustrated by considering a special family of Hubert spaces, denoted {Da}, -oo < a < oo, in the unit disc. (The values a = 0, 1 give, respectively, H2 and the Dirichlet space.) These spaces (for 0 < a < 1) were considered by Carleson in his dissertation [7]. This section contains 10 propositions and raises 6 questions, mostly for a general Banach space of analytic functions. For example, Question 3 asks if / must be cychc whenever we have \f(z)\> \g(z)\ for some cychc g, and all zl Question 4 asks if / must be cychc whenever / and 1// are both in the space. No examples are known where either of these questions has a negative answer.In §2 we begin the study of cychc vectors in the Dirichlet space D. Theorems 1 and 2 give a partial answer to Question 3 above, for this space. This section also contains 2 propositions (10,11) and 4 questions (7-10). Proposition 11 says that if/ and g are bounded functions in D whose product is cychc, then both / and g must be cychc. Theorem 2 gives a partial converse (we require that \g\ be Dini continuous on

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Section: |[K -mentioning

confidence: 92%

“…If/ e E, then zf £ E. 6. To each point weSG (the boundary of G) there corresponds a function/in E that has a singularity at w.…”

Section: Cyclic Vectors In the Dirichlet Space By Leon Brown And Allementioning

confidence: 99%

Section: Corollary M(e) C £ N Hx(g)mentioning

confidence: 99%

Section: Corollarymentioning

confidence: 99%

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Cyclic Vectors in the Dirichlet Space

Brown¹,

Shields²

1984

Transactions of the American Mathematical Society

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“…For example, Mergeljan and Tamadjan [10] (cf. also [3]) have shown that if sufficiently many slits are put in the unit disk, one can obtain a domain G such that the polynomials are dense in L 2 a (G). Once again, Corollary 2 of [16] shows that the disk with these slits is not the image of a weak-star generator of H°°.…”

Section: (S) Extends To a Function / E L^(int K)mentioning

confidence: 99%