Mantel's Theorem Theorem (Mantel 1907) If G(V,E)has IV=n and is triangle-free,then n2 E≤ 4 For n is even, extremal graph: K受，号Mantel’s Theorem Theorem (Mantel 1907) |E| ￾ n2 4 . If G(V,E) has |V|=n and is triangle-free, then For n is even, extremal graph: K n 2 , n 2