By,Originally Posted bymathlg

we mean the group of positive integers added modulo .

I believe the theorem goes that,

has a solution in when, .

For your problem you need to find all the integers relatively prime to and less. This is called the 'phi-function'. Thus, you need to find the smalles such as all the previous integers are relatively prime to it. Meaning a prime number, the smallest after 20 is 23.