本文首先给出了一类具有无穷多个周期解的无阻尼二阶线性偏微分方程所描述的系统。

This paper is devoted to study the existence of an infinitude of periodic solutions for a class of second order linear PDE systems without damping.

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六年来，我们用无阻尼水平摆地倾仪，在1.7秒—5秒的周期范围内，在一系列的强震发生前，分别记录到了一种明显的波动异常。

Since 1976, by using the non—damping horizontal pendulum tiltmeter at the range of 1.7"—5" period. we have recorded one wave anomaly signal before a series of strong earthquakes respectively.

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