audio feedback

[duffbeerforme]

hi all,I was wondering if there was an "easy" way to calculate the frequency of feedback from an amp and mic. Making it simplistic by assuming the amp and mic are linear (or is it because of nonlinearity that you get a defined frequency) and are points separated by some distance x and the mic would be fed with some random start noise (like a yelp).
My only idea was to use eulers method to simulate the situation. (easier said then done)