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# ISExpand
Computes the union of two index sets, by concatenating 2 lists and removing duplicates.
## Synopsis
```
#include "petscis.h"
PetscErrorCode ISExpand(IS is1, IS is2, IS *isout)
```
Collective
## Input Parameters
- ***is1 -*** first index set
- ***is2 -*** index values to be added
## Output Parameter
- ***isout -*** `is1` + `is2` The index set `is2` is appended to `is1` removing duplicates
## Notes
Negative values are removed from the lists. This requires O(imax-imin)
memory and O(imax-imin) work, where imin and imax are the bounds on the
indices in `is1` and `is2`.
`is1` and `is2` do not need to be sorted.
The operations are performed separately on each MPI rank
## See Also
[](sec_scatter), `IS`, `ISDestroy()`, `ISView()`, `ISDifference()`, `ISSum()`, `ISIntersect()`
## Level
intermediate
## Location
src/vec/is/is/utils/isdiff.c
## Examples
src/ksp/ksp/tutorials/ex81.c
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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/vec/is/is/utils/isdiff.c)
[Index of all IS routines](index.md)
[Table of Contents for all manual pages](/manualpages/index.md)
[Index of all manual pages](/manualpages/singleindex.md)