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Partial sums of Taylor series on a circle

E. S. Katsoprinakis, V. N. Nestoridis (1989)

Annales de l'institut Fourier

We characterize the power series n = 0 c n z n with the geometric property that, for sufficiently many points z , | z | = 1 , a circle C ( z ) contains infinitely many partial sums. We show that n = 0 c n z n is a rational function of special type; more precisely, there are t and n 0 , such that, the sequence c n e int , n n 0 , is periodic. This result answers in the affirmative a question of J.-P. Kahane and furnishes stronger versions of the main results of [Katsoprinakis, Arkiv for Matematik]. We are led to consider special families of circles C ( z ) with...

Periodic quasiregular mappings of finite order.

David Drasin, Swati Sastry (2003)

Revista Matemática Iberoamericana

The authors construct a periodic quasiregular function of any finite order p, 1 < p < infinity. This completes earlier work of O. Martio and U. Srebro.

Pick-Nevanlinna interpolation on finitely-connected domains

Stephen Fisher (1992)

Studia Mathematica

Let Ω be a domain in the complex plane bounded by m+1 disjoint, analytic simple closed curves and let z 0 , . . . , z n be n+1 distinct points in Ω. We show that for each (n+1)-tuple ( w 0 , . . . , w n ) of complex numbers, there is a unique analytic function B such that: (a) B is continuous on the closure of Ω and has constant modulus on each component of the boundary of Ω; (b) B has n or fewer zeros in Ω; and (c) B ( z j ) = w j , 0 ≤ j ≤ n.

Pointwise inequalities of logarithmic type in Hardy-Hölder spaces

Slim Chaabane, Imed Feki (2014)

Czechoslovak Mathematical Journal

We prove some optimal logarithmic estimates in the Hardy space H ( G ) with Hölder regularity, where G is the open unit disk or an annular domain of . These estimates extend the results established by S. Chaabane and I. Feki in the Hardy-Sobolev space H k , of the unit disk and those of I. Feki in the case of an annular domain. The proofs are based on a variant of Hardy-Landau-Littlewood inequality for Hölder functions. As an application of these estimates, we study the stability of both the Cauchy problem...

Polynomials, sign patterns and Descartes' rule of signs

Vladimir Petrov Kostov (2019)

Mathematica Bohemica

By Descartes’ rule of signs, a real degree d polynomial P with all nonvanishing coefficients with c sign changes and p sign preservations in the sequence of its coefficients ( c + p = d ) has pos c positive and ¬ p negative roots, where pos c ( mod 2 ) and ¬ p ( mod 2 ) . For 1 d 3 , for every possible choice of the sequence of signs of coefficients of P (called sign pattern) and for every pair ( pos , neg ) satisfying these conditions there exists a polynomial P with exactly pos positive and exactly ¬ negative roots (all of them simple). For d 4 this is not...

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