We raise the ante, by explaining why the heuristic in Chapter 1 prima facie fails. The explanation requires some surgery theory, which will be important throughout the remainder of the book, and facts about lattices, from a variety of sources.

While random matrices give us the exact value of the constant C(n), it is natural to search for alternate deterministic constructions that show that C(n)<n. This chapter explores this direction. The central notion here is that of spectral gap. To prove the key estimate that C(n)<n, it suffices to produce sequences of n-tuples of unitary matrices exhibiting a certain kind of spectral gap. The notion of quantum expanders naturally enter the discussion here. Their existence can be derived from that of groups with property (T) admitting sufficiently many finite dimensional unitary representations. The notion of quantum spherical code that we introduce hereis a natural way to describe what is needed in the present context.

Here we describe an example of group that is shown using Kazhdan’s property (T) to be such that its full C* algebrafails LLP, although the group is approximately linear (i.e. so-called hyperlinear). Since both amenable and free groups satisfy the latter LLP, it is not easy to produceexamples failing the LLP, and so far this is the only one.