With the subjunctive mood, ει' commonly takes ἄν (Epic εἴ κε or εἴ κεν, not ἐά_ν).
a. There are three forms, ἐά_ν, ἤν, ἄ_ν. ἐά_ν is the ordinary form in Attic prose and inscriptions; ἤν appears in Ionic and in the older Attic writers (the tragic poets and Thucydides); ἄ_ν, generally in the later writers (sometimes together with ἐά_ν), very rarely in Attic inscriptions. In Plato ἄ_ν is commoner than ἐά_ν. Xenophon has all three forms.
b. ἤν is from εἰ ἄν, ἄ_ν from η' (another form of εἰ) + ἄν. The etymology of ἐά_ν is uncertain: either from η' ἄν or from εἰ ἄ_ν.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].