**petsc-master 2014-09-16**

# KSPFischerGuessCreate

Implements Paul Fischer's initial guess algorithm Method 1 and 2 for situations where a linear system is solved repeatedly
### Synopsis

PetscErrorCode KSPFischerGuessCreate(KSP ksp,PetscInt method,PetscInt maxl,KSPFischerGuess *itg)

### References

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940020363_1994020363.pdf
Notes: the algorithm is different from the paper because we do not CHANGE the right hand side of the new
problem and solve the problem with an initial guess of zero, rather we solve the original new problem
with a nonzero initial guess (this is done so that the linear solver convergence tests are based on
the original RHS.) But we use the xtilde = x - xguess as the new direction so that it is not
mostly orthogonal to the previous solutions.

These are not intended to be used directly, they are called by KSP automatically when the
KSP option KSPSetFischerGuess(KSP,PetscInt,PetscInt) or -ksp_guess_fischer <int,int>

Method 2 is only for positive definite matrices, since it uses the A norm.

This is not currently programmed as a PETSc class because there are only two methods; if more methods
are introduced it should be changed. For example the Knoll guess should be included

**Level:**advanced

Location:src/ksp/ksp/interface/iguess.c

Index of all KSP routines

Table of Contents for all manual pages

Index of all manual pages