Abstract
We study the asymptotic expansion for the Landau constants \(G_n\),
where \(N=n+3/4, \gamma =0.5772\ldots \) is Euler’s constant, and \((-1)^{s+1}\beta _{2s}\) are positive rational numbers, given explicitly in an iterative manner. We show that the error due to truncation is bounded in absolute value by, and of the same sign as, the first neglected term for all nonnegative \(n\). Consequently, we obtain optimal sharp bounds up to arbitrary orders of the form
for all \(n=0,1,2,\ldots , m=1,2,\ldots \), and \(k=1,2,\ldots \). The results are proved by approximating the coefficients \(\beta _{2s}\) with the Gauss hypergeometric functions involved and by using the second-order difference equation satisfied by \(G_n\), as well as an integral representation of the constants \(\rho _k=(-1)^{k+1}\beta _{2k}/(2k-1)!\).
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References
Alzer, H.: Inequalities for the constants of Landau and Lebesgue. J. Comput. Appl. Math. 139, 215–230 (2002)
Brutman, L.: A sharp estimate of the Landau constants. J. Approx. Theory 34, 217–220 (1982)
Cvijović, D., Klinowski, J.: Inequalities for the Landau constants. Math. Slovaca 50, 159–164 (2000)
Falaleev, L.P.: Inequalities for the Landau constants. Sib. Math. J. 32, 896–897 (1991)
Granath, H.: On inequalities and asymptotic expansions for the Landau constants. J. Math. Anal. Appl. 386, 738–743 (2012)
Ismail, M.E.H., Li, X., Rahman, M.: Landau constants and their \(q\)-analogues. Anal. Appl. (2014) doi:10.1142/S0219530514500201
Landau, E.: Abschätzung der koeffzientensumme einer potenzreihe. Arch. Math. Phys. 21(42–50), 250–255 (1913)
Li, Y.-T., Liu, S.-Y., Xu, S.-X., Zhao, Y.-Q.: Full asymptotic expansions of the Landau constants via a difference equation approach. Appl. Math. Comput. 219, 988–995 (2012)
Mortici, C.: Sharp bounds of the Landau constants. Math. Comput. 80, 1011–1018 (2011)
Nemes, G.: Proofs of two conjectures on the Landau constants. J. Math. Anal. Appl. 388, 838–844 (2012)
Nemes, G., Nemes, A.: A note on the Landau constants. Appl. Math. Comput. 217, 8543–8546 (2011)
Olver, F.W.J.: Asymptotics and Special Functions. Academic Press, New York (1974)
Olver, F.W.J., Lozier, D.W., Boisvert, R.F., Clark, C.W.: NIST Handbook of Mathematical Functions. Cambridge University Press, New York (2010)
Shivakumar, P.N., Wong, R.: Asymptotic expansion of the Lebesgue constants associated with polynomial interpolation. Math. Comput. 39, 195–200 (1982)
Watson, G.N.: The constants of Landau and Lebesgue. Q. J. Math. Oxf. Ser. 1, 310–318 (1930)
Wong, R.: Lecture Notes on Applied Analysis. World Scientific, Singapore (2010)
Wong, R., Li, H.: Asymptotic expansions for second-order linear difference equations. J. Comput. Appl. Math. 41, 65–94 (1992)
Wong, R., Wyman, M.: The method of Darboux. J. Approx. Theory 10, 159–171 (1974)
Zhao, D.: Some sharp estimates of the constants of Landau and Lebesgue. J. Math. Anal. Appl. 349, 68–73 (2009)
Acknowledgments
The authors are grateful to Prof. R. Wong for bringing the problem to their attention. The authors thank the anonymous referees for their careful reading of the manuscript and for their valuable suggestions and comments. One referee suggested using a quadratic transformation of the hypergeometric functions which makes the proof of Lemma 2 more natural and simplified. The other referee provided many constructive suggestions and corrections which have much improved the readability of the manuscript. The work of Yutian Li was supported in part by the HKBU Strategic Development Fund, a start-up Grant from Hong Kong Baptist University, and a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKBU 201513). The work of Saiyu Liu was supported in part by Hunan Natural Science Foundation under Grant No. 14JJ6030 and by the National Natural Science Foundation of China under Grant No. 11326082. The work of Shuaixia Xu was supported in part by the National Natural Science Foundation of China under Grant No. 11201493, GuangDong Natural Science Foundation under Grant No. S2012040007824, and the Fundamental Research Funds for the Central Universities under Grant No. 13lgpy41. Yuqiu Zhao was supported in part by the National Natural Science Foundation of China under Grant Nos. 10471154 and 10871212.
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Communicated by Edward B. Saff.
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Li, Y., Liu, S., Xu, S. et al. Asymptotics of Landau Constants with Optimal Error Bounds. Constr Approx 40, 281–305 (2014). https://doi.org/10.1007/s00365-014-9259-x
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DOI: https://doi.org/10.1007/s00365-014-9259-x