The Laasonen Method or BI

More points might of course be chosen for the approximation, which extends the number of terms on the right-hand side, but for brevity we keep to the three-point approximation here. In Sect. 8.4 however, a model using four-point approximations is presented. 

The diffusion equation, discretised on the right-hand side as in (8.8), is now a system of odes in the concentration vector C, of the form 

When applied to the solution of odes, the BI method (Chap. 4) uses a backward difference for the derivative on the left-hand side of (8.9) and the argument of the function on the right-hand side is the future, unknown, concentration vector. In our notation, at the point i along the row of concentrations, this is 

The solution of the above system of equations (8.11) will be described below, together with that for the CN method. 

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