A SIMPLE question
 

Ive been reading through Malalasekera's and Versteeg's Introduction to Computational Fluid Dynamics through Finite Volume Method. I'm confused with how they present the SIMPLE method.

They say that a coefficient 'a' times the velocity equals the summation of the surrounding cells 'a' times velocity minus a pressure term and a source term.

au = Sum(a*u) - (p-p)/Area + Source*Volume

Sorry I don't know how to put subscripts for the i and j values.

What are the 'a' terms??? They say that they are a combination of the F and D terms that they defined earlier and can be solved for with QUICK, but they only say "a combination," rather than stating what combination.