An Improved Interpolation Theorem and Disproofs of Two Conjectures on 2-Connected Subgraphs

Abstract

We prove that any \(2\)-connected graph \(G\) on \(n\) vertices with minimum degree \(δ(G) \ge \frac{n}{4}+2\) contains a \(2\)-connected subgraph of order \(k\) for every integer \(k\) with \(4 \le k \le n\). This improves a previous result of Yin and Wu. In \cite{YinWu-DAM-2026}, Yin and Wu proposed two conjectures. The first states that for any \(2\)-connected graph \(G\) of order \(n\) and size \(m\), there exists a \(2\)-connected subgraph of order \(k\) for each \(k \in \{4, \dots, n\}\) whenever \(m \ge \frac{1}{2} n^{3/2}\). The second conjecture asserts that the same conclusion holds under the alternative condition \(δ(G) \ge \sqrt{n}\). In this paper, we construct counterexamples that completely disprove the first conjecture. Furthermore, using the existence of \((v, k, 2)\)-Symmetric Balanced Incomplete Block designs (i.e., SBIBDs), we disprove the second conjecture for all \(n \in \{8, 14, 22, 32, 74, 112, 158\}\). Finally, we propose a conjecture of our own: for any \(2\)-connected graph \(G\) on \(n\) vertices with \(δ(G) \ge \frac{n}{k}\), where \(k \ge 3\) and \(n\) is sufficiently large, \(G\) contains a \(2\)-connected subgraph of every order from \(4\) to \(n\).

Figures (4)

• Figure 1: A counterexample to Conjecture \ref{['MinSize3/2']}
• Figure 2: A hypercube $Q_3=C_4 \Box K_2$
• Figure 5: The Hanoi graph $H_3^2$
• Figure 6: A figure for Theorem \ref{['Thm:4.1']}

Theorems & Definitions (17)

• Theorem 1.1: Schuster schuster1983interpolation
• Theorem 1.2: Bondy BONDY197180
• Corollary 1.3
• Theorem 1.4
• Conjecture 1
• Conjecture 2: Yin and Wu YinWu-DAM-2026
• Theorem 1.5
• Theorem 2.1
• Lemma 2.2: Colbourn and Dinitz CD06
• Theorem 2.3