# A little help with this Taylor expansion please?

I have this translation operator T(a) that acts on a function y(x) and causes the transformation T(a)y(x) = y(x+a).
I am supposed to be "expanding y(x+a) as a taylor series in a" to show that T(a)=eipa, where p is the operator p = -i.d/dx]
So, I've started out with the general equation for the Taylor expansion, to expand, say, f(x) about the point x=a:
$$f(x) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(x=a)}{n!} (x-a)^n$$

I've used this plenty of times before but only when I have been expanding something like f(x) rather than, say, f(x+c), so I'm making mistakes and messing up when I try and work through this problem.
Anyway, back to the original thing I'm trying to expand out. What I've got so far is:

$$y(x+a) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}y(x+a = ?)}{n!}[(x+a)- ?]^n$$

Where I've left the question marks is where I'm not certain what to use. My y(x) expansion is otherwise the same way as the general formula for f(x) just with x+a replacing f(x).
I know I'm supposed to end up with something of the same form as $$e^x = \sum^{\infty}_{0} \frac{x^n}{n!} = 1 + x + \frac{x^2}{2} + ...$$ but I just can't seem to get there.

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Dick
Homework Helper
The taylor expansion of y(x+a) is the sum over n of D_n(y(x))*a^n/n! where D_n(y(x)) denotes the nth derivative of y evaluated at x, isn't it? Compare that with the operator expansion of exp(ipa) applied to y(x).

Thanks. This does work apparently:

$$Y(x+a) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}Y(x)}{n!}a^n = \frac{1}{0!}a^0Y(x) + \frac{1}{1!}a^1\frac{d}{dx}Y(x) + \frac{1}{2!}a^2\frac{d^2}{dx^2}Y(x) + ...$$
$$= Y(x) + a\frac{d}{dx}Y(x) + \frac{1}{2!}a^2\frac{d^2}{dx^2}Y(x) + ...$$
$$= Y(x) + aipY(x) + \frac{1}{2}(aip)^2Y(x) + ...$$
$$= Y(x) + aipY(x) - \frac{1}{2}a^2p^2Y(x) + ...$$
$$= (1 + aip - \frac{1}{2}a^2p^2 + ...)Y(x)$$
$$= e^{iap}Y(x) = T(a)Y(x)$$ where $$e^{iap} = 1 + aip - \frac{1}{2}a^2p^2 + ...$$ and $$p = -i(d/dx)$$

However, I'm not sure why this is. I would not have been able to do this if you hadn't told me. Certainly this sort of thing would shaft me in an exam (which I have soon). How do we get from the basic Taylor series expansion for, say, y(x), to the one that you gave me for y(x+a)? If it's too much to be bothered typing out, a link to a webpage would be nice.

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Dick
Homework Helper
$$f(x) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(a)}{n!} (x-a)^n$$
Interchange a and x since you want the derivatives evaluated at x,
$$f(a) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(x)}{n!} (a-x)^n$$
Now you don't want f(a) you want f(x+a), so put change a->x+a.
$$f(x+a) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(x)}{n!} (a)^n$$
There's lot's of ways to write the taylor series.

$$f(x) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(a)}{n!} (x-a)^n$$
$$f(a) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(x)}{n!} (a-x)^n$$
$$f(x+a) = \sum^{\infty}_{0} \frac{\frac{d^n}{dx^n}f(x)}{n!} (a)^n$$