Q: look at the ii-V-I that I attached, the key is C Major. What is going on over the V chord, G13? I see two arpegios, E major and the other of Db major Why?
A: Yes, this is a trick for the half/whole diminished scale. Actually four triads placed minor 3rds from the root of the dominant chord work well. For example, a G7 chord: G, Bb, Db and E placed over the G7 chord are all good choices for arpeggios. Obviously G placed over G7 would just be the root, 3rd and 5th of the chord. Bb would be the #9th, 5th and b7th. Db would be the b5th, b7th and b9 and E would be the 13th, b9th and 3rd. These are pretty common arpeggios to play over the G7 chord. The sound is from the diminished scale creating a 13b9 and 13#9 tonality. Check here: diminished scale