title: MPI Triangular Topology
body: <p>I have some computation to do over an (upper-)triangular matrix and I was thinking using MPI for that. It seems that it would be convenient to define a proper topology for that. The Cartesian topology goes in that direction but not quite what I need yet. I would need a topology that gives me the rank of the process on the strict upper triangular matrix, i.e. strictly above the diagonal.</p>
  
  <p>The type of triangular topology I would need would look as follows (say for 3x3 cartesian grid): </p>
  
  <pre><code>i       j      rank     
  0       0      NULL       
  0       1       1 
  0       2       2
  1       0      NULL       
  1       1      NULL
  1       2       3
  2       0      NULL       
  2       1      NULL    
  2       2      NULL
  </code></pre>
  
  <p>Thanks for your help! </p>