View source for Analytic continuation

An analytic continuation is when a function that normally converges in a [[disk of convergence]] or a [[half plane]] can be extended to a [[meromorphic function]] (or, in some cases, a [[holomorphic function]]). For example, the function <math>\sum_{n=1}^{\infty}{ar^{n}}</math> converges for <math>\mid r  \mid <1</math>. In the complex plane, this makes a circle of radius 1 centered at (0,0). This is often referred to as a [[disk of convergence]]. Inside the disk, this particular function is equal to <math>\frac{a}{1-n}</math>. We can now define it as the analytic continuation and treat it as an extension of the original function, so in this example, we might find that <math>\sum_{n=1}^{\infty}{2^n} = \frac{1}{1-2} = -1</math>. Analytic continuations are used with the [[Riemann zeta function]], which allows us many interesting results, such as <math>\sum_{n=1}^{\infty}{n} = \frac{-1}{12}</math> and <math>\sum_{n=1}^{\infty}{n^2} = 0</math>. Interestingly, even though these properties seem to be only in pure mathematics, they are often used in many areas of [[theoretical physics]], especially [[string theory]].