Before revision:  Even though we focus on wave equations in this paper, the proposed argument is directly applicable to other evolution problems|\mathcal{R}(\Delta t\lambda)|}\leq1\geq1  .
After revision:  Even though we focus on wave equations in this paper, the proposed argument is directly applicable to other evolution problems|\mathcal{R}(\Delta t\lambda)|}\leq1. For any linear multistep method, such a bound becomes c\alpha/(1-c\alpha), where c\geq1 .