Sharp constants in the Hardy-Littlewood-Sobolev and related inequalities by Elliott H. Lieb (1983)

Publisher record: annals.math.princeton.edu/1983/119-2/p05

Primary paper: www1.cmc.edu/pages/faculty/MONeill/Math%20138/papers138/Lieb.pdf

Annals of Mathematics 118, 349–374. The PDF byline spells Elliott with two t's. Cited passages: introduction and Theorem 1.1, exponent relation and bilinear Hardy-Littlewood-Sobolev inequality. The publisher URL contains 119, but the article record and PDF identify volume 118.