Friday, 21 August 2015

logarithms - Prove that logX0

I'm working through Data Structures and Algorithm Analysis in C++, 2nd Ed, and problem 1.7 asks us to prove that logX<X for all X>0.



However, unless I'm missing something, this can't actually be proven. The spirit of the problem only holds true if you define several extra qualifiers, because it's relatively easy to provide counter examples.




First, it says that logaX<X for all X>0, in essence.



But if a=1, then (1)2=1. Therefore log11=2. Thus, we must assume
a is positive.



if a is <1, then a2<1. Therefore we must assume that a1.



Now, the book says that unless stated otherwise, it's generally speaking about base 2 for logarithms, which are vital in computer science.




However, even then - if a is two and X is 116, then logaX is 4. (Similarly for base 10, try taking the log of 110 on your calculator: It's 1.) Thus we must assume that X1.



...Unless I'm horribly missing something here. The problem seems quite different if we have to prove it for X1.



But even then, I need some help solving the problem. I've tried manipulating the equation as many ways as I could think of but I'm not cracking it.

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