Preferential attachment (BA model)
Growing network: new node connects to existing with probability proportional to degree. Rich-get-richer. Produces γ = 3.
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Robustness
Robust to random failure (mostly low-degree fail). Vulnerable to targeted attack (removing hubs).
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Real examples
Web hyperlinks (γ ≈ 2.1). Internet AS graph. Airline networks. Protein interactions.
Alternative mechanisms
Copying model. Duplication + divergence. Fitness-based preferential attachment.