How to notate the final element in a sequence?

Question

I'm having troubles putting this in to words here, but here it goes: If I have a sequence of numbers, called $A$ where $A$ is a sequence of numbers that don't seem to have a pattern, how can I notate the last element in the sequence without knowing how many elements are in the sequence?
For Example: If I have $A = \langle 3, 1, 5, \ldots, 9, 5, 4\rangle$, I do not know how many elements there are but I know that last element is 4. Is there any way to notate this 4 in the sequence?

Answer 1

You are just lacking a way to express the number of elements in the sequence, it seems. You can use $\#A$ or $|A|$ or state that the sequence has length $N$. Then append the suffix $^{th}$.

The Python programming language uses the following indexing convention: elements are numbered $0$ to $N-1$, but negative indexes can be used. The element $A_{-1}$ is the last one, $A_{-2}$ the beforelast one and so on until $A_{-N}$.