<p>As far as I know, the 500 level and beyond tend to correlate with graduate level courses.</p>
<p>I’m in CAAM, so it’s a little different from MATH, but some of the classes (Real Analysis, Numerical Analysis, and electives) are usually taken by both undergrad and grad students. I haven’t seen a “graduate students only” bar on registration. </p>
<p>If you can’t register online, just talk to the prof and make sure s/he’s ok with you taking a graduate level class, and get him/her to sign the “add course” sheet, and voila, you’re in.</p>
<p>In essence, you should still be able to take it, assuming you have all the pre-requisites done.</p>