On the limiting procedure by which SDiff(T²) and SU(infty) are associated

There have been various attempts to identify groups of area-preserving diffeomorphisms of 2-dimensional manifolds with limits of SU(N) as $N\to\infty$. We discuss the particularly simple case where the manifold concerned is the two-dimensional torus $T^2$ and argue that the limit, even in the basis commonly used, is ill-behaved and that the large-N limit of SU(N) is much larger than $SDiff(T^2)$.