“някога”, in Речник на българския език[Dictionary of the Bulgarian Language] (in Bulgarian), Sofia: Bulgarian Academy of Sciences, 2014
Georgiev, Vladimir I., Duridanov, I. V., editors (1995), “някога”, in Български етимологичен речник[Bulgarian Etymological Dictionary] (in Bulgarian), volume 4 (мѝнго² – па̀дам), Sofia: Prof. Marin Drinov Pubg. House, →ISBN, page 728