Daily Archives: June 4, 2017

Jun4

Solving Logarithmic Equations

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While reading John Derbyshire’s Prime Obsession I came across the following statement (clearly explained on pp. 74):

Any positive power of \log(x) eventually increases more slowly than any positive power of x.

It is easy to prove this (existence) analytically, by taking derivative to compare slopes. But algebraically it implies that (for example):

There are either no real solution or two real solutions of the equation
\log(x) = x^\varepsilon
for any given \varepsilon>0.

Now the question that arises is “How to find this x?” I had no idea about how to solve such logarithmic equations, so I took help of Google and discovered this Mathematic.SE post. So, we put \log(x)=y and re-write the equation as:

y=e^{y\varepsilon}

Now to be able to use Lambert W function (also called the product logarithm function) we need to re-write the above equation, but I have failed to do so. 

But using WolframAlpha I was able to solve \log(x)=x^2 to get x=e^{\frac{-W(-2)}{2}} (which is an imaginary number, i.e. no real solution of this equation) but I was not able to figure out the steps involved. So if you have any idea about the general method or the special case of higher exponents, please let me know.