da vs turns: change avg to trap, use trapezoidal rule for integration (a_i=(3/2,1,...,1,3/2) instead of before a_i=1)
da vs turns: change error in amplitude for integration rule from delta r -> delta r/2
do not take last 3 divisors instead of last 2, otherwise the number of angles is smaller than 3
choose only divisors so that number of angles >3