<p>I would like to chime in a bit as I know a few professors at Yale (I spoke to them at conferences or when they gave talks at my school, and some were former professors at my school) and I know a good portion of the faculty at Yale and what it's specialties are.</p>
<p>I think Yale UG is definitely not as good as Yale grad. Yale has some really heavy applied math/comp sci people on their department, maybe 4 or 5. Yale in total I believe has less than 40 tenured professors. Some are world famous and are definitely pioneers in their field. They are particularly strong in algebra, algebraic geometry, Riemannian geometry and representation theory (actually fantastic in Representation Theory).</p>
<p>I have heard from some people who go to Yale for UG (not for math however) that indeed Yale does not seem to stack up to other comparable name schools like Harvard, Stanford, etc. I mean at Stanford, Richard Schoen did an undergrad thesis with an undergraduate math major on general relativity (Richard Schoen is a pioneer in mathematical relativity as he proved the Positive Mass Theorem with S.T. Yau his former thesis advisor). Harvard math kids routinely get publications. </p>
<p>However, like others have said, let's not get silly. Yale isn't quite the best UG math program, but it's certainly in the upper echelon and is an overall excellent math program</p>