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Proceedings of the National Academy of Sciences of the United States of America logoLink to Proceedings of the National Academy of Sciences of the United States of America
. 1981 Aug;78(8):4661–4662. doi: 10.1073/pnas.78.8.4661

Rubel's universal differential equation

R J Duffin 1
PMCID: PMC320216  PMID: 16593068

Abstract

Fourth-order differential equations such as 16y′my2 - 32ymyny′ + 17y03 = 0 are developed. It is shown that the equation is “universal” in the sense that any continuous function can be approximated with arbitrary accuracy over the whole x axis by a solution y(x) of the equation. This solution is a piecewise polynomial of degree 9 and of class C4.

Keywords: approximation theory, polynomial splines

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