# Preconditioners for MLFMM

A few preconditioners are available for the multilevel fast multipole method (MLFMM).

Note: It is recommended to use the default settings. Changing the preconditioner is recommended
only for advanced users.

On the Solve/Run tab, in the
Solution Settings group, click the Solver Settings icon. On the Solver Settings dialog, click the
Advanced tab.

- Incomplete LU decomposition (ILU) (128)
- Preconditioner for an MLFMM solution that uses an incomplete LU-decomposition of the matrix. Note, not available for parallel.
- Multilevel FEM-MLFMM LU/diagonal decomposition (2010)
- Preconditioner for a hybrid MLFMM / FEM solution that uses a multilevel sparse LU decomposition
of the combined and partitioned system.
When using the parallel Solver, the factorisation type, which slightly impacts runtime and memory, can be specified.

- Block-Jacobi preconditioner of MLFMM one-level-up (4096)
- Preconditioner for an MLFMM solution that uses LU-decomposition on entries in the boxes, one-level-up from the finest boxes. Note, not available for parallel.
- Sparse approximate inverse (SPAI) (8192)
- Preconditioner for an MLFMM solution. This
preconditioner uses less memory than the default in most cases, but runtime could be
longer.Tip: Switch to this preconditioner if the default one fails.

- Sparse LU (8193)
- Preconditioner for an MLFMM solution that uses a
sparse LU decomposition of the MLFMM near field
matrix.
When using the parallel Solver, the factorisation type, which slightly impacts runtime and memory, can be specified.

Note: The number in brackets corresponds to the value of the I

_{3}field at the CG card in the model .pre file.