Thursday, June 23, 2016

algebra precalculus - Proving an inequality log and e



This inequality should be fairly easy to show. I think I'm just having trouble looking at it the right way (It's used in a proof without explanation).


(11log2n)(2logn)1e2/logn


Any help is much appreciated. Thanks Edit: Log is base 2


Answer



Assuming n>2 are natural numbers and log=log2:


(1+1n)n is increasing, lim and (1+\frac 1x)^x < e for x \ge 1.


And (1-\frac 1x)^x > \frac 1e for x \ge 1.


So (1 - \frac 1{\log^2 n})^{\log^2 n} > e^{-1}


( 1 - \frac 1{\log^2 n})^{2\log n} > e^{\frac{-2}{\log n}}


( 1 - \frac 1{\log^2 n})^{2\log n - 1} > \frac {e^{\frac{-2}{\log n}}}{1 - \frac 1{\log^2 n}}



If n > 2 and \log n = \log_2 n > 1 then 0< {1 - \frac 1{\log^2 n}} < 1 and


( 1 - \frac 1{\log^2 n})^{2\log n - 1} > \frac {e^{\frac{-2}{\log n}}}{1 - \frac 1{\log^2 n}}>e^{\frac{-2}{\log n}}


If n = 2 then


( 1 - \frac 1{\log^2 n})^{2\log n - 1} =


( 1 - \frac 1{\log^2 2})^{2\log 2 - 1} = 0^0 is undefined.


Likewise if n=1 we have division by 0.


Perhaps \log = \ln =\log_e?


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