Why are Ramanujan sums the same as the complex Zeta values?

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ddd123
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Possibly a difficult question, but I've never found a discussion on the topic.

Thanks
 
Not much else to say, two examples:

1 + 2 + 3 + 4 + · · · = −1/12

1 + 1 + 1 + 1 + · · · = -1/2

This results both from Zeta regularization and Ramanujan summation. The two methods seem to me to be completely unrelated. It's an amazing coincidence so there must be a link between the two.

Another question would be: is it always true? But I'm assuming it is, or that the answer to the first question would answer this one too.
 
Last attempt!

Thanks