The oldest is ∗yēk- ( ἔ ŋκα, iēcē), after which ∗dhēk- ( ἔ ΰŋκα, αδδακ∈τ, fēcī) was patterned in pro-ethnic times, ἔ δωκα was added to the group only in Greek. The peculiar grouping of forms with and without -κ- in the Greek paradigms starts from the fact that forms from three bases, ∗sē-, ∗yēk-, ∗wī- (ΐŋμι, ἔ ŋκα, fi∈μαι, for instance), merged in one paradigm as a result of changes that are in part analogic, in part phonetic
Walter Petersen (Mon,) studied this question.