# PCFieldSplitSetGKBDelay
Sets the delay in the lower bound error estimate in the generalized Golub-Kahan bidiagonalization in `PCFIELDSPLIT` preconditioner. 
## Synopsis
```
#include "petscpc.h" 
PetscErrorCode PCFieldSplitSetGKBDelay(PC pc, PetscInt delay)
```
Collective on pc


## Input Parameters

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



## Options Database Key

- ***-pc_fieldsplit_gkb_delay -*** default is 5





## Note
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.


## See Also
 `PCFIELDSPLIT`, `PCFieldSplitSetGKBNu()`, `PCFieldSplitSetGKBTol()`, `PCFieldSplitSetGKBMaxit()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/fieldsplit/fieldsplit.c.html#PCFieldSplitSetGKBDelay">src/ksp/pc/impls/fieldsplit/fieldsplit.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/pc/impls/fieldsplit/fieldsplit.c.html#PCFieldSplitSetGKBDelay_FieldSplit">PCFieldSplitSetGKBDelay_FieldSplit in src/ksp/pc/impls/fieldsplit/fieldsplit.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/ksp/pc/impls/fieldsplit/fieldsplit.c)


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