The odd numbers can be generated by adding 2 to the first odd number.
Starting with 1 as the first odd number, you get 1, 3, 5, 7, 9, ...
A general formula to name the odd numbers is 2n + 1, where n can take any whole value.
Look:
n 2n + 1
0 0+1 = 1
1 2(1) + 1 = 2 + 1 = 3
2 2(2) + 1 = 4+1 = 5
3 3(2) + 1 = 6 + 1 = 7
4 4(2) + 1 = 8 + 1 = 9.
So, you now can use the general formula 2n + 1 to generate any odd number, starting with n = 0 to generate the first odd number, 1.