Principal branch of the logarithm

  • Thread starter Thread starter mxmt
  • Start date Start date
  • Tags Tags
    Branch Logarithm
Click For Summary
SUMMARY

The discussion centers on defining an analytic branch of the logarithm, specifically for the function \( w = \log\left(\frac{z+i}{z-i}\right) \). The branch cut is established as \((- \infty, 0)\), and the participant initially miscalculated \( f(1) \) as zero, later correcting it to \( f(1) = e^{i\pi/2} \). This correction is based on the mapping of the line segment from \( z = i \) to \( z = -i \) onto the logarithm's branch cut.

PREREQUISITES
  • Understanding of complex analysis and analytic functions
  • Familiarity with logarithmic functions in the complex plane
  • Knowledge of branch cuts and their implications
  • Proficiency in manipulating complex numbers and exponential forms
NEXT STEPS
  • Study the properties of complex logarithms and their branches
  • Learn about branch cuts in complex analysis
  • Explore the mapping of complex functions and their geometric interpretations
  • Investigate the implications of analytic continuation in complex functions
USEFUL FOR

Students and professionals in mathematics, particularly those focusing on complex analysis, as well as anyone interested in the properties of logarithmic functions in the complex plane.

mxmt
Messages
6
Reaction score
0

Homework Statement



Define an analytic branch f(z) of w, such that f(z)=0 for the limit of z->\infty

Now what is f(1)?

Homework Equations



w=\frac{z+i}{z-i}

The Attempt at a Solution



The branch cut of the logarithm is: (-\infty,0)
All branches of the logarithm are:
f(z)=Log(z)+iArg(z)=Log(z)+2i\pik

But then f(1)=0, which is wrong.
 
Last edited:
Physics news on Phys.org
There was a typo in my first post:

mxmt said:
w=\frac{z+i}{z-i}

Of course this should be w=log(\frac{z+i}{z-i})

Anybody who understands it now?
 
A guess is that the line segment of z=(i,-i) is mapped onto the normal branch cut of the logarithm (-inf,0). Therefore, f(1)=exp(iPi/2) because this is where i is located in the complex plane.
 

Similar threads

Replies
8
Views
3K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 14 ·
Replies
14
Views
4K
  • · Replies 5 ·
Replies
5
Views
2K
Replies
12
Views
3K
  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 16 ·
Replies
16
Views
2K