Propagatable vs propagable?

Question

propagatable vs propagable

Which one is correct? I've seen both in usage.

Answer 1

Since propagable appears in dictionaries, it might be considered more useful and appropriate in formal or technical writing:

adjective

capable of being propagated

_{Collins Dictionary}

It is derived directly from the Latin propago and the suffix -able:

1560s, "to cause to multiply,"
from Latin propagatus, past participle of propagare "to set forward, extend, procreate"
(see propagation).

mid-15c., from Old French propagacion "offshoot, offspring" (13c.) and
directly from Latin propagationem (nominative propagatio) "a propagation, extension, enlargement," noun of action
from past participle stem of propagare "set forward, extend, spread, increase; multiply plants by layers, breed,"
from propago (genitive propaginis) "that which propagates, offspring,"
from pro- "forth" (see pro-) + *pag-, root of pangere "to fasten" (see pact).

_{etymonline.com emphasis added}

Propagatable is a back formation from the English propagate, and for informal or colloquial speech, the audible clue of propagate may help less educated listeners discern the meaning. Propagatable has become as recognizable as the equally rare propagable:

_{Image from Google Books}