Wednesday, 5 June 2019

Limits of sequences connected with real and complex exponential

Let us denote Sn(x)=1+x1!+x22!+...+xnn!.





  1. How could be calculated the limit
    L(x)=lim


  2. Similar question, when x\ge 0 above is replaced by z\in \mathbb{C} ?


  3. More general, if r_{n} is a sequence of real numbers with r^{n}\searrow 1, what are the limits \lim_{n\to \infty}\frac{S_{n}(n r_{n}x)}{e^{n x}} for x\ge 0 and \lim_{n\to \infty}\frac{S_{n}(n r_{n}z)}{e^{n z}} for z\in \mathbb{C} ? (Are they equal with the limits from the above points 1) and 2) ?




Initially, my intuition told me that probably that the limit L(x) is equal to one, for any x\ge 0. But thinking better, my opinion is that the limit L(x)=0, for all x\ge 0. In support to this guess, for example, for x=1 I have calculated \frac{S_{n}(n)}{e^{n}} for several consecutive values of n and it appeared to me that it forms a decreasing sequence. In the general case, I have tried to use the Stolz-Cesaro lemma to the ratios \frac{S_{n}(nx)}{e^{n x}} and \frac{e^{nx}}{S_{n}(nx)}, but it did not work. Also, I have tried to estimate |S_{n}(nx)-e^{nx}| by using the Lagrange form of the remainder for Taylor series, but again I was not able to get any conclusion.In the complex case, the situation seems to be more intricated. Indeed, for z=i, we get \frac{S_{n}(n i)}{e^{n i}}=\frac{S^{(cos)}_{n}(n)+iS^{(sin)}_{n}(n)}{cos(n)+isin(n)}, where S^{(cos)}_{n} and S^{(sin)}_{n} represents the partial sums of order n from the series development of cosine and sine functions. The limit with n\to \infty in this case looks more tricky, as do not exist the limits \lim_{n\to \infty}cos(n) and \lim_{n\to \infty}sin(n).

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