ThisvolumeoftheEncyclopaediacontainstwoarticleswhichgiveasurveyofmodernresearchintonon-regularRiemanniangeometry,carriedoutmostlybyRussianmathematicians.
ThefirstarticlewrittenbyReshetnyakisdevotedtothetheoryoftwo-dimensionalRiemannianmanifoldsofboundedcurvature.ConceptsofRiemanniangeometrysuchastheareaandintegralcurvatureofasetandthelengthandintegralcurvatureofacurvearealsodefinedforthesemanifolds.SomefundamentalresultsofRiemanniangeometryliketheGauss.Bonnetformulaaretrueinthemoregeneralcaseconsideredinthebook.
ThesecondarticlebyBerestovskijandNikolaevisdevotedtothetheoryofmetricspaceswhosecurvatureliesbetweentwogiyenconstants.ThemainresultiSthatthesespacesareinfactRiemannian.ThisresulthasimportantapplicationsinglobalRiemanniangeometry.
Bothpartscovertopicswhichhavenotyetbeentreatedinmonographform.Hencethebookwillbeimmenselyusefultograduatestudentsandresearchersingeometry,inparticularRiemanniangeometry.