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# PCFieldSplitSetGKBDelay

Sets the delay in the lower bound error estimate in the generalized Golub-Kahan bidiagonalization preconditioner.

### Synopsis

    #include "petscpc.h"
    PetscErrorCode PCFieldSplitSetGKBDelay(PC pc,PetscInt delay)

Collective on PC

### Notes

The algorithm uses a lower bound estimate of the error in energy norm as stopping criterion. The lower bound of the error ||u-u^k||_H is expressed as a truncated sum. The error at iteration k can only be measured at iteration (k + delay), and thus the algorithm needs at least (delay + 1) iterations to stop. For more details on the generalized Golub-Kahan bidiagonalization method and its lower bound stopping criterion, please refer to

[Ar13] Generalized Golub-Kahan bidiagonalization and stopping criteria, SIAM J. Matrix Anal. Appl., Vol. 34, No. 2, pp. 571-592, 2013.

### Input Parameters

| pc | - the preconditioner context
| delay | - the delay window in the lower bound estimate

### Options Database

| -pc_fieldsplit_gkb_delay | - default is 5

### See Also

PCFIELDSPLIT, PCFieldSplitSetGKBNu(), PCFieldSplitSetGKBTol(), PCFieldSplitSetGKBMaxit()

### Level

intermediate

### Location

src/ksp/pc/impls/fieldsplit/fieldsplit.c

### Implementations

PCFieldSplitSetGKBDelay_FieldSplit in src/ksp/pc/impls/fieldsplit/fieldsplit.c
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