IA measures where in vector A (which has just been constructed) the next row starts, so to speak. The ith component of IA gives the index in A of the number that begins the ith row. If this doesn't make sense for you, consider trying to re-construct the original matrix from the Yale representation. I'm not sure myself what the last component of IA means. In the wiki example, the 0, 2, and 4 make sense; the 6 has me mystified. Another few examples would probably clear that up. This clears things up a bit, I think.