Von Neuman在1940年提出了平方取中法，其原理是将一个给定的2n比特的数平方，然后高位补0得到4n比特的数，再截取中间的2n位进行下次迭代，并得到一个随机数序列。这种方法无论是在硬件实现上，还是在仿真测试上都比较容易实现，但是它的周期受初始值的影响很大，如果初始值选得不好，会严重影响随机数序列的质量和周期，而且其最大周期不会超过22n
随机数生成
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