Table 2. Statistics of the empirical citation networks: is the number of vertices in the network; is the average indegree of the network; is the average attractiveness, determined from the tests of linear preferential attachment discussed in the text.

	1950	1955	1960	1965	1970	1975	1980	1985	1990	1995	2000	2005	2008
	15880	23350	30996	42074	62382	85590	108794	138206	180708	238142	305570	386569	441595
	2.2	3.1	3.7	4.3	5.1	5.6	6.0	6.2	6.5	7.0	7.7	8.5	9.0
	4.2	5.3	6.2	5.4	7.2	7.9	7.8	9.0	7.4	7.3	6.8	6.4	7.0