  The preferential attachment model is a natural and popular random graph model
for a growing network that contains very well-connected ``hubs''. We study the
higher-order connectivity of such a network by investigating the topological
properties of its clique complex. We concentrate on the expected Betti numbers,
a sequence of topological invariants of the complex related to the numbers of
holes of different dimensions. We determine the asymptotic growth rates of the
expected Betti numbers, and prove that the expected Betti number at dimension 1
grows linearly fast, while those at higher dimensions grow sublinearly fast.
Our theoretical results are illustrated by simulations. (Changes are made in
this version to generalize Proposition 14 and to streamline proofs. These
changes are shown in blue.)
