Lewy's example

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In the mathematical study of partial differential equations, Lewy's example is a celebrated example, due to Hans Lewy, of a linear partial differential equation with no solutions. It shows that the analog of the Cauchy–Kovalevskaya theorem does not hold in the smooth category.

Excerpt from the Wikipedia article “Lewy's example”, under the CC BY-SA 4.0 license. Read the article on Wikipedia