MathematikaPub Date : 2023-03-07DOI: 10.1112/mtk.12192
Graziano Crasta, Ilaria Fragalà
{"title":"On a geometric combination of functions related to Prékopa–Leindler inequality","authors":"Graziano Crasta, Ilaria Fragalà","doi":"10.1112/mtk.12192","DOIUrl":"10.1112/mtk.12192","url":null,"abstract":"<p>We introduce a new operation between nonnegative integrable functions on <math>\u0000 <semantics>\u0000 <msup>\u0000 <mi>R</mi>\u0000 <mi>n</mi>\u0000 </msup>\u0000 <annotation>$mathbb {R}^n$</annotation>\u0000 </semantics></math>, that we call <i>geometric combination</i>; it is obtained via a mass transportation approach, playing with inverse distribution functions. The main feature of this operation is that the Lebesgue integral of the geometric combination equals the geometric mean of the two separate integrals; as a natural consequence, we derive a new functional inequality of Prékopa–Leindler type. When applied to the characteristic functions of two measurable sets, their geometric combination provides a set whose volume equals the geometric mean of the two separate volumes. In the framework of convex bodies, by comparing the geometric combination with the 0-sum, we get an alternative proof of the log-Brunn–Minkowski inequality for unconditional convex bodies and for convex bodies with <i>n</i> symmetries.</p>","PeriodicalId":18463,"journal":{"name":"Mathematika","volume":"69 2","pages":"482-507"},"PeriodicalIF":0.8,"publicationDate":"2023-03-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1112/mtk.12192","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46898849","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
MathematikaPub Date : 2023-02-17DOI: 10.1112/mtk.12190
Joshua Zahl
{"title":"Unions of lines in \u0000 \u0000 \u0000 R\u0000 n\u0000 \u0000 $mathbb {R}^n$","authors":"Joshua Zahl","doi":"10.1112/mtk.12190","DOIUrl":"10.1112/mtk.12190","url":null,"abstract":"<p>We prove a conjecture of D. Oberlin on the dimension of unions of lines in <math>\u0000 <semantics>\u0000 <msup>\u0000 <mi>R</mi>\u0000 <mi>n</mi>\u0000 </msup>\u0000 <annotation>$mathbb {R}^n$</annotation>\u0000 </semantics></math>. If <math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>d</mi>\u0000 <mo>⩾</mo>\u0000 <mn>1</mn>\u0000 </mrow>\u0000 <annotation>$dgeqslant 1$</annotation>\u0000 </semantics></math> is an integer, <math>\u0000 <semantics>\u0000 <mrow>\u0000 <mn>0</mn>\u0000 <mo>⩽</mo>\u0000 <mi>β</mi>\u0000 <mo>⩽</mo>\u0000 <mn>1</mn>\u0000 </mrow>\u0000 <annotation>$0leqslant beta leqslant 1$</annotation>\u0000 </semantics></math>, and <i>L</i> is a set of lines in <math>\u0000 <semantics>\u0000 <msup>\u0000 <mi>R</mi>\u0000 <mi>n</mi>\u0000 </msup>\u0000 <annotation>$mathbb {R}^n$</annotation>\u0000 </semantics></math> with Hausdorff dimension at least <math>\u0000 <semantics>\u0000 <mrow>\u0000 <mn>2</mn>\u0000 <mo>(</mo>\u0000 <mi>d</mi>\u0000 <mo>−</mo>\u0000 <mn>1</mn>\u0000 <mo>)</mo>\u0000 <mo>+</mo>\u0000 <mi>β</mi>\u0000 </mrow>\u0000 <annotation>$2(d-1)+beta$</annotation>\u0000 </semantics></math>, then the union of the lines in <i>L</i> has Hausdorff dimension at least <math>\u0000 <semantics>\u0000 <mrow>\u0000 <mi>d</mi>\u0000 <mo>+</mo>\u0000 <mi>β</mi>\u0000 </mrow>\u0000 <annotation>$d + beta$</annotation>\u0000 </semantics></math>. Our proof combines a refined version of the multilinear Kakeya theorem by Carbery and Valdimarsson with the multilinear → linear argument of Bourgain and Guth.</p>","PeriodicalId":18463,"journal":{"name":"Mathematika","volume":"69 2","pages":"473-481"},"PeriodicalIF":0.8,"publicationDate":"2023-02-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1112/mtk.12190","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44163916","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}