In the context of the SIS (Susceptible–Infected–Susceptible) model applied to complex networks, which of the following statements correctly describes the behavior of the epidemic threshold \( \lambda_c \) in scale-free networks with a degree exponent \( 2 < \gamma < 3 \), compared to random networks?
a) The epidemic threshold is identical in both types of networks, depending only on the average degree \( \langle k \rangle \) and the recovery rate, ensuring robustness against viruses with a low spreading rate.
b) Scale-free networks possess an extremely high epidemic threshold due to the presence of hubs, making it very difficult for an epidemic to spread unless the transmission rate is exceptionally high.
c) In scale-free networks, in the limit of large networks \( N \rightarrow \infty \), the second moment of the degree distribution diverges \( \langle k^2 \rangle \rightarrow \infty \). As a consequence, the epidemic threshold approaches zero \( \lambda_c = 0 \).
d) Random immunization is the most effective strategy to increase the epidemic threshold in scale-free networks, outperforming targeted immunization of high-degree nodes.
e) None of the above.
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