Henri Leon Lebesgue (1875 - 1941) was a French mathematician, noted for his theory of integration; see Lebesgue integration.
[I have moved the rest of the content to the discussion page. On the one hand, it is clear that the author of this page knows some Lebesgue integration theory. On the other hand, there is already a page on Lebesgue integration, and most of the material belongs there rather than in an article about the person. The TeX is not correctly done. It should be like this: