Sequence Notation

You write sequences using subscript notation. The general term is $a_n$ where $n$ is the position (starting from $1$).

For the sequence $3, 6, 9, 12, ...$, you can write $a_n = 3n$. This formula tells you the $n$-th term: $a_1 = 3(1) = 3$, $a_2 = 3(2) = 6$, and so on.

Having a formula for $a_n$ lets you find any term directly without computing all previous terms. This is like array indexing with a formula instead of storage.