△-free→lEl≤n2/4 Third Proof. A:maximum independent set a lAl independent v∈V,d≤a dv B=V\A B incident to all edges B=B ⊙和 Inequality of the arithmetic and geometric mean B 因≤sa时s(2）f- 4 u∈B-free 㱺 |E| ≤ n2/4 Third Proof. A: maximum independent set B = V \ A α = |A| β = |B| v ⇤ ⇥￾ ⌅ dv independent ⇤v ⇥ V, dv ￾ ￾ B B incident to all edges ￾ ￾⇥ ￾ ￾￾ + ⇥ 2 ⇥2 |E| ￾ ￾ v￾B dv = n2 4 Inequality of the arithmetic and geometric mean