向量范数。http://baike.baidu.com/link?url=L8LBQ6qYkfPsIgHEG6G58KvlsG1gvW26swsqTRLqMPfIaO7yZVXln1ejt5QRvCejvmUDsGGbcd5xSMMIoMsF3q#3