The terms in the corresponding series are constructed by adding consecutive single numbers.
The term 13 is added to the first two odd numbers, 1,3.
3 , add the next single number 5 and add 35
5, add the next ouncum, 7, and 57
If you continue to strike like this, 9,
The next term will be 911 by adding 11, which is a single number after 9.