Improving Hölder's inequality

International audienceWe show that the remainder in Hölder's inequality (Rogers, 1888; Hölder, 1889) may be computed exactly. It satisfies functional equations, and possesses monotonicity and scaling properties. We obtain as a consequence improvements of recent sharpenings (Aldaz, 2008) of the classical inequality

Boundary blow-up and degenerate equations

AbstractLet Ω⊂R2 be a bounded domain of class C2+α, 0<α<1. We show that if u is the solution of Δu=4exp(2u) which tends to +∞ as (x,y)→∂Ω, then the hyperbolic radius v=exp(−u) is also of class C2+α up to the boundary. The proof relies on new Schauder estimates for degenerate elliptic equations of Fuchsian type

Brahmagupta's derivation of the area of a cyclic quadrilateral

International audienceThis paper shows that Propositions XII.21–27 of Brahmagupta's Br¯ ahma-sphut. asiddh¯ anta (628 a.d.) constitute a coherent mathematical discourse leading to the expression of the area of a cyclic quadrilateral in terms of its sides. The radius of the circumcircle is determined by considering two auxiliary quadrilaterals. Observing that a cyclic quadrilateral is split by a diagonal into two triangles with the same circumcenter and the same circumradius, the result follows, using the tools available to Brahmagupta. The expression for the diagonals (XII.28) is a consequence. The shortcomings of earlier attempts at reconstructing Brahmagupta's method are overcome by restoring the mathematical consistency of the text. This leads to a new interpretation of Brahmagupta's terminology for quadrilaterals of different types. Résumé. On montre que les propositions XII.21–27 du Br¯ ahmasphut. asiddh¯ anta (628 ap. J.-C.) forment un discours cohérent conduisantàconduisantà l'expression de l'aire d'un quadrilatère cyclique en termes de ses côtés. Le rayon du cercle circonscrit est déterminé en considérant deux quadrilatères auxiliaires. Exprimant que le quadrilatère cyclique est partagé par une diagonale en deux triangles ayant en commun le centre et le rayon de leur cercle circonscrit, on obtient l'aire du quadrilatère, ` a l'aide des outils connus de Brahmagupta. L'expression des diagonales (XII.28) en découle. Les difficultés des tentatives antérieures en vue de retrouver la démarche de Brahmagupta sont résolues en restituant la cohérence mathématique du texte. On est ainsi conduitàconduità une nouvelle interprétation des termes qu'utilise Brahmagupta pour désigner des quadrilatères de différentes classes

The initial singularity of ultrastiff perfect fluid spacetimes without symmetries

We consider the Einstein equations coupled to an ultrastiff perfect fluid and prove the existence of a family of solutions with an initial singularity whose structure is that of explicit isotropic models. This family of solutions is `generic' in the sense that it depends on as many free functions as a general solution, i.e., without imposing any symmetry assumptions, of the Einstein-Euler equations. The method we use is a that of a Fuchsian reduction.Comment: 16 pages, journal versio

Fuchsian analysis of singularities in Gowdy spacetimes beyond analyticity

Fuchsian equations provide a way of constructing large classes of spacetimes whose singularities can be described in detail. In some of the applications of this technique only the analytic case could be handled up to now. This paper develops a method of removing the undesirable hypothesis of analyticity. This is applied to the specific case of the Gowdy spacetimes in order to show that analogues of the results known in the analytic case hold in the smooth case. As far as possible the likely strengths and weaknesses of the method as applied to more general problems are displayed.Comment: 14 page