  The percolation threshold of the network model by Barabasi and Albert
(BA-model) [Science 286, 509 (1999)] has thus far only been 'guessed' based on
simulations and comparison with other models. Due to the still uncertain
influence of correlations, the reference to other models cannot be justified.
In this paper, we explicitly derive the well-known values for the BA-model. To
underline the importance of a null model like that of Barabasi and Albert, we
close with two basic remarks. First, we establish a connection between the
abundance of scale-free networks in nature and the fact that power-law tails in
the degree distribution result only from (at least asymptotically) linear
preferential attachment: Only in the case of linear preferential attachment
does a minimum of topological knowledge about the network suffice for the
attachment process. Second, we propose a very simple and realistic extension of
the BA-model that accounts for clustering. We discuss the influence of
clustering on the percolation properties.
