How to find the join condition (SQL)?

Question

Algorithms

The input is a table t with one column.

f1
1
2
3
4
5
6
7
8
9

When combined with itself, we get

f1  f2
1   1
2   1
3   1
4   3
5   3
6   3
7   5
8   5
9   5

That is, f2 increases by 2 every 3 values ​​of f1
Question. How to find the condition of such a connection?
And what to read to be able to make such conditions?

Answer 1

How to find the condition of such a connection?

Easily. f2 = int((f1-1)/3)*2+1

And what to read to be able to make such conditions?

School course of mathematics.