# How did this expansion take place?

1. Jan 26, 2013

### M. next

Ok suppose we have e$^{x}$, by Taylor expansion it becomes $\sum$ x$^{n}$/n!
Then it was set equal to: $\sum$ x $^{2n}$/2n! + $\sum$ x $^{2n+1}$/(2n+1)!

Note: summation in all was from zero to infinity..
How did this take place??
Thanks

2. Jan 26, 2013

### cepheid

Staff Emeritus
All it's doing is splitting the series into its even and odd terms. Think about it: for n = 0, the first expression gives you x0/0! and the second one gives you x1/1!

For n=1, the first expression gives you x2/2! And the second expression gives you x3/3!

Et cetera...

Between the two new series, all the terms from the original Taylor series are included.

3. Jan 26, 2013

### M. next

Thank you a lot for this reply, but please bear with me and explain this sentence, " Between the two new series, all the terms from the original Taylor series are included."
How come?

Last edited by a moderator: Jan 26, 2013
4. Jan 26, 2013

### cepheid

Staff Emeritus
Because the first series has all the even terms from the original series, and the second one had all the odd terms from the original series.

I wrote out the first few terms for you already, hoping to illustrate this. TRY IT for yourself. Write out a few more terms.

5. Jan 26, 2013

### M. next

Yes, you did. Thank you a lot :)!

6. Jan 26, 2013

### jbunniii

By the way, beware that in general you cannot split an infinite series into its even and odd terms like that, without potentially changing the answer. It is only guaranteed to work if the series is absolutely convergent. Therefore some justification is needed before making manipulations like that. Fortunately, all power series (including Taylor series) are absolutely convergent within the interior of their radius of convergence. But convergence may only be conditional for points at the radius itself.

7. Jan 26, 2013

### M. next

Thank you for the information. I will be careful when dealing with those.