Thanks for both answers. Knowing the term bandwidth reduction helps a lot finding information on internet!
And for the diagonal of a non-square matix.. just the line from the top left to the bottem right... :-)
When I have a matrix like this:
0 0 1 0
0 1 0 0
1 0 0 1
0 0 1 1
0 0 0 1
I can get most non zero values in the centre changing only the order of rows
1 0 0 1
0 1 0 0
0 0 1 0
0 0 1 1
0 0 0 1
Probably when I am allowed to change the order of columns and rows at the same time...