, it is instead cast into "delta" form. Defining 
, the system to be solved becomes:
Rather than solving Eqn. 376 for the unknowns , it is instead cast into "delta" form. Defining , the system to be solved becomes:
| (377) |
The right-hand side:
| (378) |
is termed the residual, and represents the discretized form of the original equation (Eqn. 336) at iteration k. By definition, then, the residual will be zero when the discretized equation is satisfied exactly.