A mathematical problem of finding the law in the first day of junior high school Serial number 1 2 3 4 5 6... N Number 1 3 7 15 127? Add context

A mathematical problem of finding the law in the first day of junior high school Serial number 1 2 3 4 5 6... N Number 1 3 7 15 127? Add context

2-1
4-2
8-1
……
2^n-1