Many years ago when I was studying for the second year in the university of Jyväskylä in one sleepless night I somehow invented how to determine logarithm of negative real number, although I had only basic knowledge of complex numbers. And yes, the logarithm of negative real number is, of course, a complex number.

Now to the formula…

Let x ∈ ℜ and *x* < 0 and *k* > 1. Now

I was able to prove this formula in less than 30 minutes, about one A4 paper of proof. Next morning I showed my proof to one person at staff of department of mathematics at university of Jyväskylä (Finland). He didn’t find anything wrong in my proof. A tricky part was a situation where there was two variables in one equation in my proof, but the other varibale had as coefficient sin π that equals 0, so the other variable was eliminated from the equation. I don’t have the proof anymore and I don’t really remember any relevant details of my proof other than I used Euler’s formula (*) in some part of the proof that I had become familiar with in course of differential equations.

(*) As far I can see, the formula probably was the following:

e^{x}^{ + iy }= e^{x}(cos *y* + *i* sin *y*)

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