Reddit mentions of Arvo Pärt (Oxford Studies of Composers)

"Für Alina" is a great starting point to see how this works. The right-hand part is freely composed and does not have an obvious pattern, but the left-hand part is algorithmically generated from the right-hand part (except for one note). First you might try transcribing it from a recording, or you can look at the score if you have it. Then, think of it this way: imagine you have a RH part already freely written, and you want to assign one note to the LH for each note in the right hand. What note will it be? (What are the only pitches that the LH part plays? How does the RH determine which one of those possibilities is chosen?)

If you're curious, I ran across this in Paul Hillier's book on Pärt, but you can probably find other resources online. I like this one for Fratres.