Walsh–Lebesgue theorem
The Walsh–Lebesgue theorem is a famous result from harmonic analysis proved by the American mathematician Joseph L. Walsh in 1929, using results proved by Lebesgue in 1907.[1][2][3] The theorem states the following: Let K be a compact subset of the Euclidean plane ℝ2 such the relative complement of [math]\displaystyle{ K }[/math] with respect to ℝ2 is connected. Then, every real-valued continuous function on [math]\displaystyle{ \partial{K} }[/math] (i.e. the boundary of K) can be approximated uniformly on [math]\displaystyle{ \partial{K} }[/math] by (real-valued) harmonic polynomials in the real variables x and y.[4]
Generalizations
The Walsh–Lebesgue theorem has been generalized to Riemann surfaces[5] and to ℝn.
This Walsh-Lebesgue theorem has also served as a catalyst for entire chapters in the theory of function algebras such as the theory of Dirichlet algebras and logmodular algebras.[6]
In 1974 Anthony G. O'Farrell gave a generalization of the Walsh–Lebesgue theorem by means of the 1964 Browder–Wermer theorem[7] with related techniques.[8][9][10]
References
- ↑ Walsh, J. L. (1928). "Über die Entwicklung einer harmonischen Funktion nach harmonischen Polynomen". J. Reine Angew. Math. 159: 197–209. http://eudml.org/doc/149665.
- ↑ Walsh, J. L. (1929). "The approximation of harmonic functions by harmonic polynomials and by harmonic rational functions". Bull. Amer. Math. Soc. 35 (2): 499–544. doi:10.1090/S0002-9947-1929-1501495-4.
- ↑ Lebesgue, H. (1907). "Sur le probléme de Dirichlet". Rendiconti del Circolo Matematico di Palermo 24 (1): 371–402. doi:10.1007/BF03015070. https://zenodo.org/record/2189809.
- ↑ Gamelin, Theodore W. (1984). "3.3 Theorem (Walsh-Lebesgue Theorem)". Uniform Algebras. American Mathematical Society. pp. 36–37. ISBN 9780821840498. https://books.google.com/books?id=2-K2A7cdORoC&pg=PA36.
- ↑ Bagby, T.; Gauthier, P. M. (1992). "Uniform approximation by global harmonic functions". Approximations by solutions of partial differential equations. Dordrecht: Springer. pp. 15–26 (p. 20). ISBN 9789401124362. https://books.google.com/books?id=vzrsCAAAQBAJ&pg=PA20.
- ↑ Walsh, J. L. (2000). Rivlin, Theodore J.. ed. Joseph L. Walsh. Selected papers. Springer. pp. 249–250. ISBN 978-0-387-98782-8. https://books.google.com/books?id=Zm6ahqIyF5QC&pg=PA249.
- ↑ Browder, A.; Wermer, J. (August 1964). "A method for constructing Dirichlet algebras". Proceedings of the American Mathematical Society 15 (4): 546–552. doi:10.1090/s0002-9939-1964-0165385-0.
- ↑ O'Farrell, A. G (2012). "A Generalised Walsh-Lebesgue Theorem". Proceedings of the Royal Society of Edinburgh, Section A 73: 231–234. doi:10.1017/S0308210500016395. http://archive.maths.nuim.ie/staff/aofarrell/preprint/1974agwlt.pdf.
- ↑ O'Farrell, A. G. (1981). "Five Generalisations of the Weierstrass Approximation Theorem". Proceedings of the Royal Irish Academy, Section A 81 (1): 65–69. http://archive.maths.nuim.ie/staff/aof/preprint/19815gotwat.pdf.
- ↑ O'Farrell, A. G. (1980). "Theorems of Walsh-Lebesgue Type". Aspects of Contemporary Complex Analysis. Academic Press. pp. 461–467. http://archive.maths.nuim.ie/staff/aof/preprint/1980towlt.pdf.
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