


10 Introduction to factorials and combinationsWhat is very interesting, is that we will always be able to observe the same kind of constant even with the introduction of an equivalent combination or factorial (as long as we stay in a ), this is why and the logarithms will represent the essential results given by simple hypergeometric. On the other hand, as soon as we introduce a squared combination for example, we will obtain different kind, something to do with the gamma function most of the time (and so is ! but of a less obvious way).
10.1 A first exampleThe introduction of the combination is justified by several very similar series with or without the presence of combinations. For example,
Which means that in fact as soon as we find a result to this function, we have a little hope to find the same kind of result with a combinations with a central binomial coefficient for example. It's simply due to the form quite close to generators functions of those series (1211).
10.2 Umbral calculusWe can establish a link betweens the series with combinations ( more precisely) and the series without () with the help of the acceleration of the convergence by Euler's method. This is a particular case of the formula for "finite differences" which is the discrete version of Taylor's expansion (). It's the theory known under the name of ”Umbral calculus”, whose origines goes back to Sylvester (18141897). The acceleration method by Euler consist to replacing the sum of a serie by the sum of the difference in terms of the serie. More precisely, we choose an alternating serie and we calculate the finite difference of order for :
then
The trick, it's that the first term is sometime written as combinations or factorial ratio and other symbols of Pochammer ! Application : We know that . Hence we have . being quite delicate to calculate directly, we let the function
We are going to look for the value in of course. And we calculate it ! where is the complete Bêta function which is worth in this case integers (interesting...). From which we obtain and hence according to the formula 364 Unfortunatly, I have not found how to generalise this method... However, we have still the more general formula
or even
where B is an incomplete Bêta , if I'm not mistaken... Similarly, by using the link between the incomplete bêta and the hypergeometric where is the hypergeometric function. Basicly, the introduction to factorials can be interesting, very interesting....
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