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# MatShift

Computes `Y = Y + a I`, where `a` is a `PetscScalar`

## Synopsis

    #include "petscmat.h"
    PetscErrorCode MatShift(Mat Y, PetscScalar a)

Neighbor-wise Collective

## Input Parameters

- Y - the matrix

- a - the `PetscScalar`

## Notes

If `Y` is a rectangular matrix, the shift is done on the main diagonal of the matrix (https://en.wikipedia.org/wiki/Main_diagonal)

If the matrix `Y` is missing some diagonal entries this routine can be very slow. To make it fast one should initially fill the matrix so that all diagonal entries have a value (with a value of zero for those locations that would not have an entry). No operation is performed when a is zero.

To form Y = Y + diag(V) use `MatDiagonalSet()`

## See Also

Matrices, `Mat`, `MatDiagonalSet()`, `MatScale()`, `MatDiagonalScale()`

## Level

intermediate

## Location

src/mat/utils/axpy.c

## Examples

src/ksp/ksp/tutorials/ex27.c
src/ksp/ksp/tutorials/ex1.c
src/tao/pde_constrained/tutorials/hyperbolic.c
src/ksp/pc/tutorials/ex4.c
src/ksp/ksp/tutorials/ex81a.c
src/ksp/ksp/tutorials/ex79.c
src/tao/pde_constrained/tutorials/parabolic.c
src/ksp/ksp/tutorials/ex81.c
src/ksp/ksp/tutorials/ex82.c
src/tao/constrained/tutorials/tomographyADMM.c

## Implementations

MatShift_MPIAIJKokkos() in src/mat/impls/aij/mpi/kokkos/mpiaijkok.kokkos.cxx
MatShift_MPIAIJ() in src/mat/impls/aij/mpi/mpiaij.c
MatShift_SeqAIJ() in src/mat/impls/aij/seq/aij.c
MatShift_SeqAIJKokkos() in src/mat/impls/aij/seq/kokkos/aijkok.kokkos.cxx
MatShift_MPIBAIJ() in src/mat/impls/baij/mpi/mpibaij.c
MatShift_SeqBAIJ() in src/mat/impls/baij/seq/baij.c
MatShift_ConstantDiagonal() in src/mat/impls/cdiagonal/cdiagonal.c
MatShift_MPIDense() in src/mat/impls/dense/mpi/mpidense.c
MatShift_SeqDense() in src/mat/impls/dense/seq/dense.c
MatShift_Diagonal() in src/mat/impls/diagonal/diagonal.c
MatShift_IS() in src/mat/impls/is/matis.c
MatShift_Nest() in src/mat/impls/nest/matnest.c
MatShift_MPISBAIJ() in src/mat/impls/sbaij/mpi/mpisbaij.c
MatShift_SeqSBAIJ() in src/mat/impls/sbaij/seq/sbaij.c
MatShift_ScaLAPACK() in src/mat/impls/scalapack/matscalapack.c
MatShift_MPISELL() in src/mat/impls/sell/mpi/mpisell.c
MatShift_SeqSELL() in src/mat/impls/sell/seq/sell.c
MatShift_Shell() in src/mat/impls/shell/shell.c
MatShift_SubMatrix() in src/mat/impls/submat/submat.c

Index of all Mat routines
Table of Contents for all manual pages
Index of all manual pages