A strengthening of a degree sequence condition for Hamiltonicity in tough graphs

Generalizing Chv\'atal's classic 1972 result, Ho\`ang proposed in 1995 the following conjecture, which strengthens Chv\'atal's result in terms of toughness: Let $t\ge 1$ be a positive integer and $G$ be a $t$-tough graph on $n \ge 3$ vertices with degree sequence $d_1, d_2, \dots, d_n$ in non-increasing order. Suppose for each $i\in [1, \lfloor\frac{n-1}{2} \rfloor]$, if $d_i \le i \text{ and } d_{n-i+t} < n - i$ implies $d_j + d_{n-j+t} \ge n$ for all $j\in [i+1, \lfloor\frac{n-1}{2} \rfloor]$, then $G$ is Hamiltonian. Ho\`ang verified the conjecture for $t=1$. In this paper, we verfity the conjecture for all $t\ge 4$. Our proof relies on a toughness closure lemma for $t\ge 4$ that we previously established. Additionally, we show that the toughness closure lemma does not hold when $t=1$.