Sometimes, the notation is the biggest hurdle to overcome when (re)learning mathematics.
Variables and indexes
So that we can write general equations, we need to use variables to represent the quantities of interest.
Vectors
A very important structure, especially in engineering and computer science, and something we need to understand properly before going any further.
Series, summation, product
It's common for variables to occur in sequences, so we need notation for that. Then we can perform sum and product operations on them.
