How to get the residue of this funciton at z=i

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Homework Help Overview

The problem involves finding the residue of the function f(z) = cot(πz)/(z² + 1) at the point z = i. The context is within complex analysis, specifically focusing on residues and singularities.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to expand cot(πz) at z = i and seeks clarification on the formula for csc(A+B) as part of their approach. Other participants question the necessity of this expansion, suggesting that cot(πz) is analytic at z = i and that the singularity arises solely from the denominator.

Discussion Status

Participants are exploring different interpretations of the problem, with some suggesting that the original poster's approach may not be necessary. There is a recognition of the analytic nature of cot(πz) at the point of interest, which may guide the discussion towards a more straightforward understanding of the residue.

Contextual Notes

The discussion includes a focus on the properties of cotangent and the nature of singularities in the context of the function provided. There is an implicit acknowledgment of the original poster's confusion regarding the expansion methods and the role of the denominator in determining the residue.

justin_huang
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Homework Statement



f(z)=cot(pi*z)/(z^2+1)

Homework Equations





The Attempt at a Solution


now I want to get the residue at z=i, I know the definition of f(z)'s residue form
but when I try to get the expansion of cot(pi*z) at z=i, I used a lot of method
like use sin*csc this form but due to the reason I cannot get the expansion of csc(A+B)
i CANNOT get the result

Could you tell me the formula of csc(A+B) or other more easier method to figure this problem? thanks
 
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cot = cotangent = cos / sin. I don't know why you are using csc = cosecant = 1 / sin
 
SteamKing said:
cot = cotangent = cos / sin. I don't know why you are using csc = cosecant = 1 / sin

cause i need to get the multiplication of two polynomial not division
 
justin_huang said:
cause i need to get the multiplication of two polynomial not division

You don't have to expand cot(pi*z) at z=i. cot(pi*z) is analytic at z=i, it has a perfectly well defined limit there. The singularity is only in the denominator.
 
Dick said:
You don't have to expand cot(pi*z) at z=i. cot(pi*z) is analytic at z=i, it has a perfectly well defined limit there. The singularity is only in the denominator.

I got it... thanks so much
 

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