Understanding Notations for Sequences
Understanding sequences is an important first step toward understanding series. The simplest notation for defining a sequence is a variable with the subscript n surrounded by braces. For example:
You can reference a specific term in the sequence by using the subscript:
Make sure you understand the difference between notation with and without braces:

The notation {a_{n}} with braces refers to the entire sequence.

The notation a_{n} without braces refers to the nth term of the sequence.
When defining a sequence, instead of listing the first few terms, you can state a rule based on n. (This is similar to how a function is typically defined.) For example:
Sometimes, for increased clarity, the notation includes the first few terms plus a rule for finding the nth term of the sequence. For example:
This notation can be made more concise by appending starting and ending values for n:
This last example points out the fact that the initial value of n doesn’t have to be 1, which gives you greater flexibility to define a number series by using a rule.
Don’t let the fancy notation for number sequences get to you. When you’re faced with a new sequence that’s defined by a rule, jot down the first four or five numbers in that sequence. After you see the pattern, you’ll likely find that a problem is much easier.