↑ 3.03.13.23.3Cheung, Johnny (2007) “*dar¹”, in Etymological Dictionary of the Iranian Verb (Leiden Indo-European Etymological Dictionary Series; 2), Leiden, Boston: Brill, →ISBN, pages 57-59
^ Derksen, Rick (2015) “derėti”, in Etymological Dictionary of the Baltic Inherited Lexicon (Leiden Indo-European Etymological Dictionary Series; 13), Leiden, Boston: Brill, →ISBN, page 122
^ Derksen, Rick (2015) “daryti”, in Etymological Dictionary of the Baltic Inherited Lexicon (Leiden Indo-European Etymological Dictionary Series; 13), Leiden, Boston: Brill, →ISBN, page 116
^ De Vaan, Michiel (2008) “firmus”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 223
^ De Vaan, Michiel (2008) “ferē”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, page 212
^ De Vaan, Michiel (2008) “frēnum”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, pages 241-242
^ De Vaan, Michiel (2008) “frētus”, in Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7), Leiden, Boston: Brill, →ISBN, pages 242-243