Some Improved Almost Sharp Inequalities Under Constraints

Abstract In this paper, we present a new proof of the improved Moser–Trudinger–Onofri inequality under constraints, originally established by Aubin [2, 3] and further developed by Chang and Hang [6, 14] on $S^{n}$. Our approach provides deeper insight into the relationship between the optimal constant in these inequalities and the number of potential blow-up concentration points. We also extend the inequalities to non-spherical domains, including the torus, annulus, and rectangle, where similar intriguing phenomena emerge. Additionally, we formulate higher-order versions of these inequalities involving general GJMS operators. Besides, we establish a refinement of the concentration compactness principle for the Adams inequality, as well as an improved Riesz-type Adams inequality under constraints. Furthermore, we prove an improved almost sharp Sobolev inequality on general compact manifolds and establish an improved almost sharp higher-order Sobolev–Beckner inequality on $S^{n}$.