mixed matrix type?

[Sean Dettrick]

Hi,

I have a sparse N*N matrix generated from a DA and a 5 point stencil,  
with a total of approx 5*N non-zero entries.  Now I would like to  
extend this matrix by adding a smaller M*M dense matrix to the bottom  
right hand side, i.e. so that there is a dense square in the bottom  
right hand corner of the otherwise sparse matrix.  The total number of  
new non-zero entries, M*M, is comparable to the total number of old  
entries, 5*N.  On top of this, there would be a small number of non- 
zero entries in the new upper-right and lower-left rectangular  
portions of the matrix, due to coupling of the two systems.  The new  
total matrix size (including zeroes) would be (N+M)*(N+M).

Can anybody recommend a Mat type to store the new matrix?

One possibility I was thinking of was to establish the original sparse  
Mat with a DA in a sub-communicator (with half the CPUs), and get the  
ownership range with MatGetOwnershipRange.  Then in the  
petsc_comm_world communicator, the complete matrix could be  
constructed (by element-wise copying the old one I suppose), and the  
ownership range could be maintained manually.

Does this sound like a reasonable strategy?

I would very much appreciate any suggestions or advice.

Thanks,

Sean Dettrick