# Homework Help: D-operator homework help

1. Dec 2, 2011

### sharks

I'm reviewing my notes and i can't understand this part:

I have enclosed each corresponding term in the same coloured box. I've reviewed this solved example several times now but can't figure out how the result was obtained in the last line. I expanded each term (D^2 - 3D)/5 according to its power but ended up with terms exceeding powers of 3.

2. Dec 2, 2011

### I like Serena

Re: D-operator

Hi sharks!

This is an application of Taylor's theorem.
In this particular case it means that:
1/(1+x)=1-(1/x)+(1/x)^2-...

It holds if |x| is less than 1, and also, in this particular case, it is the result of the corresponding geometric series.

3. Dec 2, 2011

### Fredrik

Staff Emeritus
Re: D-operator

I think you meant
$$\frac{1}{1+x}=1-x+x^2-x^3+\cdots$$ Anyway, that explains the second line, but sharks appears to be asking about the third.

Yes, that's what the dots are for... There are infinitely many terms in the second line too. The last term before the dots in the second line is included to make sure that all terms with D3 will be included in the last line. Higher order terms in the second line (the terms represented by dots) don't contain any Dn terms with n<4.

4. Dec 2, 2011

### I like Serena

Re: D-operator

Blamey! Yes, that's what I meant.

5. Dec 2, 2011

### sharks

Re: D-operator

It's actually a Binomial series. And after some more digging around the net, i found the perfect example:
http://www.ucl.ac.uk/Mathematics/geomath/level2/series/ser84.html

Thanks for the help. I didn't know what it was, as i had just expanded the whole thing at first.

Cheers!

6. Dec 2, 2011

### I like Serena

Re: D-operator

Oh, it's a specific occurrence of the binomial series for n=-1 instead of a general n.
Note that the binomial series is a special case of Taylor's theorem.

Cheers!