# Calculate the following limit ( not sure if possible!)

1. Dec 26, 2013

### csopi

1. The problem statement, all variables and given/known data
Calculate the following limit for real $t$-s.

$$\sum_{n=0}^{∞} exp[i\cdot \sqrt{n + 1}\cdot t] / n!$$

2. Relevant equations
None

3. The attempt at a solution
Without the root it's trivial... I am not sure if it is even possible to give a closed form, I am out of ideas. Any help would be greatly appreciated!

2. Dec 26, 2013

### jbunniii

What have you tried? A reasonable first step would be to put $m = \sqrt{n+1}$. What does that give you?

3. Dec 26, 2013

### csopi

Thank you for your efforts! I've tried that, but I think it won't help, because m won't be an integer. I also tried to approximate with an intergral using Stirling's formula for n!, but the resulting intergral seems too complicated. I'm also considering to use somehow the residue theorem, but so far nothing.

4. Dec 26, 2013

### StevePMcGrew

You can build an Excel spreadsheet in about five minutes to do the calculation to a very good approximation:
For T=pi, for example, the limit is approximately Lim = .539061035756653 -i*.335197295005148.

5. Dec 26, 2013

### haruspex

Not sure whether it provides any useful clues, but here are a couple of plots. One plots y against x (i.e. complex plane), the other plots r and theta as functions of t. For the second, I normalised r by dividing by the t=0 value (e), and normalised theta by subtracting t (which seems to be the asymptotic behaviour) then dividing by pi.

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