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I am trying to solve the following limit:

$ \lim_{x \to 1^+} \frac{x^x+\frac{x}{2}-\sqrt x-\frac{1}{2}}{\log(x)-x+1} $

My attempt was to substitute $x$ with $1+y$, which results in the following:

$ \lim_{y \to 0^+}_{y<1} \frac{(y+1)^{y+1}+\frac{y}{2}-\sqrt{1+y}}{\sum_{n=1}^\infty (-1)^n\frac{y^{n+1}}{n+1}} $

I strongly suspect that the limit does not exists (the term seems to diverge towards negative infinity), but I don't know how to proceed from here. I think that it might be easiest to find a null sequence to substitute $y$ with, that shows that the values of the term above are unbounded, but I am stuck at this for some time now.

Could somebody point me into the right direction?

UPDATE: Unfortunately I am not allowed to use De L'Hopital's rule yet.

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    Are you allowed to use Taylor series?2012-01-18

4 Answers 4

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$\begin{aligned} \lim_{x \to 1^+} \frac{x^x+\frac{x}{2}-\sqrt x-\frac{1}{2}}{\ln(x)-x+1} \\&=\lim _{x\to 0^+}\left(\frac{\left(t+1\right)^{\left(t+1\right)}+\frac{t+1}{2}-\sqrt{t+1}-\frac{1}{2}}{\ln \left(t+1\right)-t}\right) \\&=\lim _{x\to 0^+}\left(\frac{1+t+o\left(t\right)+\frac{t+1}{2}-\left(1+\frac{1}{2}t+o\left(t\right)\right)-\frac{1}{2}}{t-\frac{t^2}{2}+o\left(t^2\right)-t}\right) \\&=\lim _{x\to 0^+}\left(-\frac{2}{t}+o\left(t\right)\right) \\&=\color{red}{-\infty} \end{aligned}$ Solved with Taylor expansion

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You should apply de L'Hospital rule. You will get

$ \lim_{x \to 1^+} \frac{x^x+\frac{x}{2}-\sqrt x-\frac{1}{2}}{\log(x)-x+1}= \lim_{x \to 1^+} \frac{x^x(\log(x)+1)+\frac{1}{2}-\frac{1}{2\sqrt x}}{\frac{1}{x}-1}=-\infty $

The other way is using Taylor series as follows

$ \lim_{x \to 1^+} \frac{x^x+\frac{x}{2}-\sqrt x-\frac{1}{2}}{\log(x)-x+1}= \lim_{x \to 1^+} \frac{1+(x-1)+\frac{x}{2}-1-\frac{x-1}{2}-\frac{1}{2}}{x-1-\frac{1}{2}(x-1)^2-x+1}=-\infty $

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    Thank you for this alternative solution.2012-01-18
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If you can't use L'Hopitals rule then your best bet is to use series expansion about x=1. This is basically what you're already doing with your transform $y=1+x$.

The tricky part is that you're going to have to use the binomial theorem on two of the bits on the top line:

$(1+y)^{(1+y)} = 1 + (1+y)*y + \ldots = 1 + y + y^2 + \ldots$

and

$\sqrt{1+y} = (1+y)^{1/2} = 1 + (1/2)y + \ldots$

Then your top line becomes

$ \left(1 + \frac{y}{2} + O(y^2)\right) + \frac{y}{2} - \left( 1 + \frac{y}{2} + O(y^2) \right) = \frac{y}{2} + O(y^2) $

The bottom, expaning your sum is

$-y^2 + O(y^3)$

So overall your fraction becomes:

$\frac{\frac{y}{2} + O(y^2)}{-y^2 + O(y^3) } = \frac{-1}{2y}\frac{1 + O(y)}{1 +O(y)} = \frac{-1}{2y}(1+O(y))$

Its clear that this will behave like $\frac{-1}{2y}$ for $y$ near 0.

Thus the result will be unbounded.

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Use L'Hopitals rule, which basically says:

If $\lim_{x\to1} f(x) = 0$ and $\lim_{x\to1} g(x) = 0$, then

\lim_{x\to1^+} \frac{f(x)}{g(x)} = \lim_{x\to1^+} \frac{f'(x)}{g'(x)}

You should find for your function f'(1) \neq 0 and g'(1)=0 so that the limit is unbounded.

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    That'd be possible, but we haven't proven and thus are not allowed use L'Hopital yet, unfortunately. I'll edit the answer to mention this.2012-01-18