# Why does symrcm create larger band width?

When I run the following (in Matlab) on a sparse matrix $A$, I get larger band width. The symrcm (symmetric reverse Cuthill-McKee permutation) is not guarenteed to find the smallest band width, but it makes no sense for it to increase. What is wrong here?

reorderingForSmallBand = symrcm(A);
A = A(reorderingForSmallBand,reorderingForSmallBand);


Running spy(A) on $A$ before and after reordering yields: Before (PDF) and After (PDF)

Originally, my matrix $A$ has this form (PDF). I also wonder if it is a big deal to have large bandwidth as the backslash solver in Matlab says it is above the limit for a solver exploiting the banded structure. Is it any advantage for an iterative solver to have small band width?

• Are there any function in Matlab which reduces the bandwidth of the original matrix $A$ to the first matrix? – user253249 Apr 28 '15 at 8:44