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I've got an assignment here, and one question is throwing me off, as I've never seen it written like this before..

$$\int\frac{\ln^3 x}{x}\ dx$$

Is this the same as $$\int\frac{(\ln x)^3}{x}\ dx\;\;?$$

amWhy
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Mirrana
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  • Yes it is. Moreover $$ \int \frac{\ln^3(x)}{x},{\rm d}x = \textstyle{\frac 14}ln^4(x) + c $$ – AndreasT Feb 08 '13 at 14:37
  • Yeah - I figured that would be the result if I had assumed my hypothesis was correct. Thanks for clarifying this. – Mirrana Feb 08 '13 at 14:38
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    For future reference, if you write \ln x instead of ln x it doesn't look like $l$ times $n$ times $x$. Same goes for \sin, \exp, etc. –  Feb 08 '13 at 14:59

2 Answers2

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Unfortunately, "shorthand" can lead to ambiguity:

$\ln^3 x = (\ln (\ln(\ln x))\;?\quad$ or $\quad \ln^3x = (\ln x)^3\;?$

But as you suspected, in this context, and given the integral, I'm am quite sure that $\ln^3x = (\ln x)^3,\;$ much like $\;\sin^2(x) = (\sin x)^2.\;$ So your integral amounts to:

$$\int\frac{\ln^{3}x}{x}\ dx = \int \frac{(\ln x)^3}{x} \,dx$$

Let $u = \ln(x),;\;du = \dfrac{dx}{x}$

$$\int \frac{(\ln x)^3}{x} \,dx = \int u^3\,du = \frac14 u^4 + C $$ $$= \frac14 (\ln x)^4 + C = \frac 14\ln^4x + C$$

amWhy
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Unfortunately two conventions prevail: $\ln^3 x$ can mean $\ln\ln\ln x$ or it can mean $(\ln x)^3$. The first one is reasonable; the second one is unfortunately also seen. I would write $(\ln x)^3$, or $(\log x)^3$, or $(\log_e x)^3$, if I meant $(\log_e x)^3$.

It became universally standard to construe $\sin^3 x$ to mean $(\sin x)^3$, over the objection of Carl Gauss (the most famous person to live on earth in the 19th century, except perhaps those who did not work in the physical and mathematical sciences), who said $\sin^3 x$ ought to mean $\sin\sin\sin x$. You can't push back the tide with a pitchfork.

Michael Hardy
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  • $\sin^2x=(\sin x)^2$ has a lot to answer for... I need to use $\arcsin x$ now or else I see $\sin^{-1} x=\frac{1}{\sin x}$ etc. – JP McCarthy Feb 08 '13 at 17:10