Sreznevsky, Izmail I. (1893) “къто”, inМатеріалы для Словаря древне-русскаго языка по письменнымъ памятникамъ [Materials for the Dictionary of the Old East Slavic Language Based on Written Monuments][2] (in Russian), volume 1(А – К), Saint Petersburg: Department of Russian Language and Literature of the Imperial Academy of Sciences,column1415