CFD Online Discussion Forums - What are residual related coefficients in Fluent?

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What are residual related coefficients in Fluent?

Hi,
While going over the documentation on how ANSYS Fluent calculates residuals, I came across the equation: $a_P \phi_P = \sum_{nb}a_{nb} \phi_{nb} + b$
Here,

is center coefficient and $a_{nb}$ are influence coefficients from neighboring cells. This is calculated for a general variable $\phi$ in cell P. What exactly are these coefficients?

Source:

HTML Code:

https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node812.htm