I am completely fascinated by the Euler/Ramanujan result
$$ \sum_{n=0}^{\infty} n = -\frac{1}{12}$$
It is amazing to me that there are so many seemingly bogus ways to evaluate this, and they all give the same ridiculous answer. It's magical. I want to understand what's going on here more deeply.
I have been reading up on the Euler-MacLaurin formula, Ramanujan Summation, and the Abel-Plana formula, but wikipedia does not go into much depth. Where can I find what I need to understand and prove these results? I have an undergrad degree in math, but still get quickly lost in most research-level papers.