🌐EnglishEnglishDeutschEspañolFrançaisItalianoNederlandsPortuguêsSvenskaTürkçeРусский한국어中文日本語EntityQ794042· pop 21· linked from 151 articlesBessel's inequalitytheoremConnectionsFourier seriesEntityinner product spaceEntityCategoriesFourier analysisHilbert spacesInequalities (mathematics)Theorems in functional analysis