[mywittyname]

That's the sum of all integers on the complex plane, solved using an analytical continuation of the Riemann zeta function at -1 (fwiw, by the same definition, the sum of 13, 26, 39...inf is also -1/12).

It's disingenuous to assert that is same as the sum of that infinite series without the associated caveats. By the definitions of infinite series that we all learned in calculus, that is a divergent series with no sum.

Edit: Wolfram Alpha[0] has a good graphic showing why this series converges to -1/12 (the one with the red line, drawing a peach-like shape). It also gives an intuition as to how complex numbers are influencing the results despite being omitted from the equation.

[0] http://mathworld.wolfram.com/RiemannZetaFunction.html