I found the quote from the letter to Pythocles very interesting. "But above all give yourself up to the study of first principles (τὴν τῶν ἀρχῶν) and of infinity (ἀπειρίας)..."
Άπειρος in its various forms appears around 40 times in Diogenes Laertius book 10, with many in Epicurus's writings.
Αρχή about two dozen times.
My little project will be to list those out when I get a chance to see how Άπειρος gets used and translated... Since Epicurus appears to call us to study these ideas.
I find it interesting that αρχή is a limit, the beginnings or foundations, and άπειρος is something without limits.