Successive remainders of the Newton series

Author:

Crofts G. W.,Shaw J. K.

Abstract

If f f is analytic in the open unit disc D D and λ \lambda is a sequence of points in D D converging to 0, then f f admits the Newton series expansion f ( z ) = f ( λ 1 ) + n = 1 Δ λ n f ( λ n + 1 ) ( z λ 1 ) ( z λ 2 ) ( z λ n ) f(z) = f({\lambda _1}) + \sum \nolimits _{n = 1}^\infty {\Delta _\lambda ^nf({\lambda _{n + 1}})(z - {\lambda _1})(z - {\lambda _2}) \cdots (z - {\lambda _n})} , where Δ λ n f ( z ) \Delta _\lambda ^nf(z) is the n n th divided difference of f f with respect to the sequence λ \lambda . The Newton series reduces to the Maclaurin series in case λ n 0 {\lambda _n} \equiv 0 . The present paper investigates relationships between the behavior of zeros of the normalized remainders Δ λ k f ( z ) = Δ λ k f ( λ k + 1 ) + n = k + 1 Δ λ n f ( λ n + 1 ) ( z λ k + 1 ) ( z λ n ) \Delta _\lambda ^kf(z) = \Delta _\lambda ^kf({\lambda _{k + 1}}) + \sum \nolimits _{n = k + 1}^\infty {\Delta _\lambda ^nf({\lambda _{n + 1}})(z - {\lambda _{k + 1}}) \cdots (z - {\lambda _n})} of the Newton series and zeros of the normalized remainders n = k a n z n k \sum \nolimits _{n = k}^\infty {{a_n}{z^{n - k}}} of the Maclaurin series for f f . Let C λ {C_\lambda } be the supremum of numbers c > 0 c > 0 such that if f f is analytic in D D and each of Δ λ k f ( z ) , 0 k > \Delta _\lambda ^kf(z),\;0 \leqslant k > \infty , has a zero in | z | c |z| \leqslant c , then f 0 f \equiv 0 . The corresponding constant for the Maclaurin series ( C λ {C_\lambda } , where λ n 0 {\lambda _n} \equiv 0 ) is called the Whittaker constant for remainders and is denoted by W W . We prove that C λ W {C_\lambda } \geqslant W , for all λ \lambda , and, moreover, C λ = W {C_\lambda } = W if λ l 1 \lambda \in {l_1} . In obtaining this result, we prove that functions f f analytic in D D have expansions of the form f ( z ) = n = 0 Δ λ n f ( z n ) C n ( z ) f(z) = \sum \nolimits _{n = 0}^\infty {\Delta _\lambda ^nf({z_n}){C_n}(z)} , where | z n | W |{z_n}| \leqslant W , for all n n , and C n ( z ) {C_n}(z) is a polynomial of degree n n determined by the conditions Δ λ j C k ( z j ) = δ j k \Delta _\lambda ^j{C_k}({z_j}) = {\delta _{jk}} .

Publisher

American Mathematical Society (AMS)

Subject

Applied Mathematics,General Mathematics

Reference11 articles.

1. Zeros of partial sums of power series. II;Buckholtz, J. D.;Michigan Math. J.,1970

2. Whittaker constants;Buckholtz, J. D.;Proc. London Math. Soc. (3),1971

3. Zeros of partial sums and remainders of power series;Buckholtz, J. D.;Trans. Amer. Math. Soc.,1972

4. On the convergence of the Abel-Gončarov interpolation series;Dragilev, M. M.;Uspehi Mat. Nauk,1960

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1. Zero-free disks in families of analytic functions;Lecture Notes in Mathematics;1987

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