文摘
For \({\left(C(t)\right)_{t \geq 0}}\) being a strongly continuous cosine family on a Banach space, we show that the estimate \({\limsup_{t \to 0^{+}} \|C(t) - I\| < 2}\) implies that C(t) converges to I in the operator norm. This implication has become known as the zero-two law. We further prove that the stronger assumption of \({\sup_{t \geq 0} \|C(t) - I\| < 2}\) yields that C(t) = I for all \({t \geq 0}\). For discrete cosine families, the assumption \({\sup_{n \in \mathbb{N}} \|C(n) - I\| \leq r < \frac{3}{2}}\) yields that C(n) = I for all \({n \in \mathbb{N}}\). For \({r \geq \frac{3}{2}}\), this assertion does no longer hold. Mathematics Subject Classification Primary 47D09 Secondary 47D06