Monday, 28 September 2015

Intuition behind logarithm inequality: 1frac1xleqlogxleqx1



One of fundamental inequalities on logarithm is:
11xlogxx1for all x>0,
which you may prefer write in the form of
x1+xlog(1+x)xfor all x>1.



The upper bound is very intuitive -- it's easy to derive from Taylor series as follows:
log(1+x)=i=1(1)n+1xnn(1)1+1x11=x.




My question is: "what is the intuition behind the lower bound?" I know how to prove the lower bound of log(1+x) (maybe by checking the derivative of the function f(x)=x1+xlog(1+x) and showing it's decreasing) but I'm curious how one can obtain this kind of lower bound. My ultimate goal is to come up with a new lower bound on some logarithm-related function, and I'd like to apply the intuition behind the standard logarithm lower-bound to my setting.


Answer



Take the upper bound:
lnxx1
Apply it to 1/x:
ln1x1x1
This is the same as
lnx11x.


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