Abstract index notation

Abstract index notation is a way of writing down mathematical equations that describes what we want the equation to do, without giving instructions on how to do it. It's like a set of instructions written in mathematical symbols that show what steps should be done in a certain order. For example, if we want to multiply two numbers, we could write down the equation using abstract index notation like this: A_i * B_i. The "A_i" stands for one of the numbers, and the "B_i" stands for the other number. The "_i" stands for "index", a way of saying that the numbers are in a certain order in the equation.