Summing a finite series of natural numbers

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Really we'll only treat the case starting with 1, but this can be easily extended to cover starting at some . Given a sequence of numbers , the series is given by . Finding the sum of the series is actually quite simple:

Given the series:

We can rearrange the series like so:

The question is where does it stop? That is, how many terms do we have? Obviously if there is an even number of elements in the sequence you can do this exactly times. This gives us:

But what if its odd? Well, the middle term will now be (think about that a minute and it will be obvious to you), but you only have repeated times. That gives us the following

So we see that this series always converges to the same formula.

In the above case, we have seen that represents the largest number in the sequence. If we are looking at a complete graph where represents the number of nodes (), the sequence goes from to . How will this change answer? How would it change your answer if we started at and went up to ?