Residue Theorem: Show r1+r2 = Res(f1+f2, z0)

In summary, to show that the residue of f1 + f2 is r1 + r2, we can use the fact that the residue of a function at a point is equal to the limit of (z-z0) multiplied by the function at that point. By applying this formula to f1 and f2, we can easily see that the residues of the sum of f1 and f2 is equal to the sum of their individual residues. Therefore, the residue of f1 + f2 is r1 + r2.
  • #1
SNOOTCHIEBOOCHEE
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Homework Statement



If f1 and f2 have residues r1 and r2 at z0. show that the residue of f1+ f2 is r1 + r2

The Attempt at a Solution



Res(f1, z0) = limz-->z0 (z-z0)f1(z) = r1
Res(f2, z0) = limz-->z0 (z-z0)f2(z) = r2

now calculate Res(f1+f2, z0)

=

limz-->z0 (z-z0)(f1(z)+ f2(z))
= limz-->z0 (z-z0)(f1(z))+(z-z0)(f2(z) = r1+ r2

Is it really this easy? i must be doing something wrong
 
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  • #2
Pretty much, yeah. "Residue" is only defined for poles and a function has a pole at [itex]z_0[/itex] if and only if it can be expanded in a power series (a Laurent series) with a finite number of negative exponents. In that case the residue is the coefficient of z-1 (thus that limit formula you use). Adding the two functions, you can add the Laurent series term by term: [itex]a_1z^{-1}+ a_2z^{-2}= (a_1+ a_2)z^{-1}[/itex]. The residues add.
 
  • #3
Thanks halls.
 

1. What is the Residue Theorem?

The Residue Theorem is a mathematical concept used to evaluate complex integrals. It states that the integral of a function around a closed curve is equal to the sum of the residues of the function inside the curve. It is a powerful tool in complex analysis and can be used to simplify the calculation of certain types of integrals.

2. What is a residue?

A residue is a complex number that represents the coefficient of the term with the highest negative power in the Laurent series expansion of a function. In other words, it is the coefficient of the term with the highest power of (z-z0) in the expression for the function. The residue theorem states that the residue of a function at a point z0 is equal to the integral of the function around a small closed curve centered at z0.

3. How is the residue theorem used to show r1+r2 = Res(f1+f2, z0)?

The equation r1+r2 = Res(f1+f2, z0) is a direct application of the residue theorem. It states that the sum of the residues of two functions, f1 and f2, at a point z0 is equal to the residue of the sum of the two functions, f1+f2, at the same point z0. This can be proved by using the definition of a residue and the properties of complex integrals.

4. What is the significance of r1+r2 = Res(f1+f2, z0)?

The equation r1+r2 = Res(f1+f2, z0) is significant because it allows us to simplify the calculation of complex integrals. By breaking down a complex integral into smaller, simpler integrals using the residue theorem, we can evaluate the integral more easily. This makes the residue theorem a valuable tool in solving problems in complex analysis and other related fields.

5. Are there any limitations to the residue theorem?

Yes, the residue theorem has some limitations. It can only be applied to integrals over closed curves, and the curve must enclose a finite number of singularities. It also assumes that the function is analytic everywhere except at the singularities. Additionally, the residues must be calculated accurately in order for the theorem to be applied correctly. Overall, the residue theorem is a powerful tool, but it may not be applicable in every situation.

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