We put the known numbers and the corresponding serial numbers together to observe and compare, and the common one is arithmetic progression.

Second, the common factor method

Divide the given number by the least common factor and multiply it to see if it is related to n or 2n, 3n.

Third, the first digit method

A given number is subtracted, added, multiplied or divided with the first digit at the same time to form a new series, and then the relationship with the serial number can be found, and the law can be found.

Fourth, odd and even numbers are separated.

List the odd-numbered digits and even-numbered digits respectively to form two series, and then find out the rules.