Task number: 3971
Show that each \( k \)-connected graph on at least \( 2k \) vertices contains a cycle of length at least \( 2k \).
Use the fan theorem if the longest cycle is not long enough.
If \( C \) is the longest cycle and its length is less than \( 2k \), then we choose any vertex \( v \) outside \( C \). It follows from Menger’s theorem that between \( v \) and \( C \) exist \( k \) vertex disjoint paths, some two of which lead to adjacent vertices of \( C \) (according to Dirichlet’s principle). Instead of this edge, we extend the cycle \( C \) through the paths and \( v \).