# Horribly Confused With Complex Logarithms

1. May 6, 2014

### MrBillyShears

I'm getting myself all confused with complex logarithms. I'll try to explain why. One identity with complex logarithms is ln(z^c)=cln(z)+2πik, with k an integer. This is, of course, a more general case of ln(e^c)=c+2πik, but it doesn't always work the same! Lets say we are evaluating ln(e^i). Using the latter identity, it is i+2πik, which is, logically, the correct answer, but using the first identity, you get iln(e)+2πik, which is i(1+2πin)+2πik=i+2πn+2πik...! What! Obviously e^(i+2π) doesn't equal e^i. Another example, ln(1)=ln(e^2πi)=2πi(1+2πin)+2πik=2πi+4π^2n+2 πik

And, I have another problem. I have this when I try to solve an equation 10^z=e^πi, so I take ln of both sides zln(10)=πi+2πik and then z=(πi+2πik)/ln(10), where ln(10) in the denominator is infinite answered and will give solutions that don't work! I'm clearly doing something wrong, so someone please help me!

2. May 6, 2014

### Staff: Mentor

If this identity allows complex c (check that!), then you have to be more careful which branch of the logarithm you choose on the right side.

A multi-valued log is not the exact inverse function of 10^z for reasonable definitions of 10^z. If you use the same freedom to define 10^z as exp(ln(10)z) with a multi-valued ln(10) then they should work.