Wikipedia:Reference desk/Archives/Mathematics/2015 April 20

= April 20 =

Surprising periodic series
I'm looking for a property, such that the existense of series having this property - is not surprising, yet the existence of periodic (or even just cyclic) series having this property - is very surprising, whereas... the mathematicians have really discovered such a periodic (or cyclic) series having this property ! HOOTmag (talk) 20:18, 20 April 2015 (UTC)


 * Surprising is in the eye of the beholder. For me, when I first encountered the Gibbs phenomenon, it was surprising. I could believe that a function with a jump discontinuity could be represented by a infinite series of some basis functions (e.g., the Dirac delta), but that approximating a square wave with a Fourier periodic series would result in an extra spike jump discontinuity was counterintuitive. And yet, there it is. --Mark viking (talk) 00:55, 22 April 2015 (UTC)


 * Thank you so much.
 * Your example inspired me to think about many other examples of series whose periodicity is surprising. For example, until De-Moivre's and Euler's era, everyone thought that no smooth non-permanent exponential function - can be periodic (by "exponential function" I mean a function whose value for any sum of two numbers is the product of the values of the function for both of them). Then De-Moivre and Euler surprised the world by coming up with De-Moivre's formula and Euler's formula, that involve some kind of exponential function which - despite its being smooth and non-permanent - it was (surprisingly) periodic, so that one can now define a series whose periodicity is surprising - by using (trivially) the surprising periodicity of the function defined in De-Moivre's formula and in Euler's formula ...
 * Actually, it's unnecessary to go up to De-Moivre's and Euler's modern age: Even the early man, who only knew natural numbers, could easily set up a series by stipulating its periodicity in some surprise. For example, let P be any proposition which - surprisingly - turns out to be true (e.g. "the animal sitting behind me is a tiger"). Now we define the "property" of the series S, as follows: If P is false then S is not periodic (but say converges with the identity function), while if P is true then S is periodic (say converges with the constant number zero) ...
 * HOOTmag (talk) 08:21, 22 April 2015 (UTC)