指定想要获得字符串的长度，（可选，没有指定章前往从指定开端方位到原字符串完毕方位）
The number of characters in the substring. If this argument is omitted, the returned substring includes all characters from the starting position to the end of the string.
论文第四章重点介绍用非线性映射方法分析原核生物基因密码子使用情况并得到的相关结论。
In chapter IV, the codon usage preferences in prokaryotic organism are analyzed by nonlinear mapping method.
在第二章，我们提出了一种解决具有原方块角形结构正定二次规划问题的分解协调算法。
In chapter 2, we presented a method for solving positive definite quadratic programming with coupled-block structure.
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