Abstract:We study stability forms of two classical integral inequalities: Hardy's integral inequality and the integral Carleman inequality, also known as the Pólya--Knopp inequality. The sharp constants in these two inequalities are $(p')^p$ and $e$, respectively, but neither inequality has a non-trivial extremizer in its natural function space. Their formal extremal functions are $c x^{-1/p}$ and $c/x$, respectively. We first derive a deficit identity for Hardy's integral inequality. In particular, when $p=2$, the deficit is exactly a squared norm. We then prove a distance stability estimate for the integral Carleman inequality, whose remainder term directly measures a local weighted Hellinger-type distance from the family $\{c/x:c\ge0\}$. Together these results illustrate the same stability phenomenon: although the classical integral inequalities have no genuine extremizers, their deficits still measure the deviation from the corresponding families of virtual extremals.
From: Gangsong Leng [view email]
[v1]
Sat, 13 Jun 2026 02:02:48 UTC (7 KB)
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