# How did this expansion take place?

• M. next
In summary, the conversation discusses the splitting of a Taylor series into its even and odd terms. This is done by setting the original series equal to two new series, one with all the even terms and one with all the odd terms. However, this manipulation is only valid for absolutely convergent series, which includes power series like the Taylor series. Care must be taken when dealing with series that may only be conditionally convergent at the radius of convergence.
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

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.

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?

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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.

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

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.

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

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