Past exam of the mathematics course of the University of Cambridge / 2015 / ii / Paper 3 / 14I / b / Solution

Let $v_1\cdots v_k$ be a longest path. Every neighbour of either endpoint lies on it. Among the $k-1$ indices $i=1,\dots ,k-1$, mark those with $v_1{v}_{i+1}$ an edge and those with $v_i{v}_k$ an edge. Their total number isso some index has both marks. The edges at that index close the path into the cycle graphThe minimum-degree hypothesis also forces the graph to be a connected graph: each component would otherwise have at least $n/2+1$ vertices. If $k<n$, an edge from this cycle to an outside vertex would give a longer path by opening the cycle at its incident vertex. Hence $k=n$, proving the Dirac theorem and Hamiltonicity.

The bound is sharp. For $n=2m$, take two disjoint copies of $K_m$, with minimum degree $m-1$. For $n=2m+1$, take two copies of $K_{m+1}$ sharing exactly one vertex. The minimum degree is $m$, but the shared vertex is a cut vertex, which no Hamiltonian graph can have. Both constructions have