I'm having a hard time translating this phrase from Caesar's De Bello Gallico. I understand, from doing a bit of research, that probat illis introduces indirect speech.
Perfacile factu esse illis probat conata perficere, propterea quod ipse suae civitatis imperium obtenturus esset:
(De Bello Gallico, Book I, Chapter III, Section V)
So far I have, "he proved to them that completing these efforts was done very easily". But why is factu in its peculiar form? I would expect a past participle, factum, which would agree with the case, gender and number of the infinitive perficere. I don't understand the -u ending of factu.
Thanks for any help!