Riemann zeta function
However, this doesn't apply for numbers where , since if we interpret this function as an infinite sum, the sum does not converge. Instead, it diverges. This means that instead of nearing a specific value, it will get infinitely large. Riemann used analytic continuation, so that he could give a value to all numbers except 1.  does not have a limit in that point and thus the function is not defined there.