The given series is:

$7 + 9 + 11 + 13$

We observe that the terms in the series are increasing by 2 each time. This is an arithmetic sequence, where the first term $a = 7$ and the common difference $d = 2$. The number of terms is 4.

To express this series in sigma notation, we define the general term of the sequence:

$a_n = 7 + (n-1) \cdot 2 = 2n + 5$

Now, we can write the series in sigma notation:

$\sum_{n=1}^{4} (2n + 5)$

Would you like a detailed explanation on how this form was derived?

Here are five related questions you might find interesting:

1. How would the sigma notation change if the series continued to 15 terms?
2. Can you express a general arithmetic sequence in sigma notation?
3. How do you convert a geometric series to sigma notation?
4. What is the difference between arithmetic and geometric series?
5. How would you calculate the sum of an arithmetic series without using sigma notation?

Tip: Arithmetic sequences are characterized by a constant difference between consecutive terms, which makes them easy to express in a linear form.