septenary number

例句：

1. 1. The octogenarian is a person who has lived for more than eighty years.
   八旬老人是指活了八十岁以上的人。
2. 2.The seven-day workweek is a common arrangement in many countries.
   七天工作周是许多国家常见的安排。
3. 3. The septenary number is a prime number that occurs only once every seven years.
   七次方数是一个只出现一次每七年的质数。