"Principal Branch Square Root of z in Domain C-{0}

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Homework Help Overview

The discussion revolves around the principal branch square root of z and its potential Laurent series expansion in the domain C-{0}. Participants are exploring the implications of branch points and the analyticity of the function in relation to its branch cut.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the definition and implications of the principal branch of the square root function, particularly regarding its analyticity and the existence of a Laurent series expansion in the specified domain. There is uncertainty about how the branch cut affects the function's properties.

Discussion Status

The discussion is active, with participants presenting differing views on the analyticity of the principal branch and its implications for the existence of a Laurent series. Some guidance has been offered regarding the nature of branch cuts and their impact on analyticity, but no consensus has been reached.

Contextual Notes

Participants are navigating the complexities of branch points and cuts, specifically how they relate to the analyticity of the principal branch square root function in the context of complex analysis. There is a focus on the implications of these concepts for the existence of series expansions.

ryou00730
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Homework Statement


Does the principal branch square root of z have a Laurent series expansion in the domain C-{0}?

The Attempt at a Solution



Well I'm not really sure what a principal branch is? I believe that there is a Laurent series expansion for z^(1/2) in C-{0} because originally our only problem is that when we take derivative of z^1/2 we get 1/z^[(2n+1)/2] and this is not defined at 0, but is everywhere else... so I think the answer is yes to this, but again I'm unsure of the details of principal branch?
 
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No. The square root is a multifunction and these functions do not have Laurent series about their branch-points because they're not fully analytic in a punctured disc surrounding the branch-point, specifically not so along their branch-cuts
 
But does taking the principal branch of square root z not deal with that? Does the principal branch mean we only take the principal roots of z?
 
The principal branch is analytic except along it's branch-cut which extends out from the origin so that we do not have an analytic domain in a punctured disk surrounding the origin and yes, the principal branch is the principal root with arg between -pi and pi
 

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