Exercise · ZW-RESIDUE-004 · ★★☆ Intermediate
What is the residue of \(\frac{1}{z^2+1}\) at \(z=i\)?
Choose one option.
Show answer
- C. \(-\frac{i}{2}\)
Explanation
It is \(\frac{1}{(z-i)(z+i)}\), a simple pole: the residue is \(\frac{1}{z+i}\) evaluated at \(i\), that is, \(\frac{1}{2i}=-\frac i2\).
Review the concept: Residue
The residue of a function \(f\) at an isolated singularity \(z_0\) is the coefficient \(a_{-1}\) of \((z-z_0)^{-1}\) in its Laurent series on a punctured disc of radius \(r\).
\[\operatorname{Res}(f,z_0)=a_{-1}=\frac{1}{2\pi i}\oint_{|z-z_0|=r/2}f(z)\,dz\]
More practice problems: residue
- ZW-RESIDUE-001 What is the residue of (1)/(z^2) at z=0?
- ZW-RESIDUE-002 What is the residue of e^1/z at z=0?
- ZW-RESIDUE-003 What is the residue of (1)/(z(z-1)) at z=0?
- ZW-RESIDUE-005 What is the residue of (e^z)/((z-1)^2) at z=1?
- ZW-RESIDUE-006 What is the residue of (sin z)/(z^4) at z=0?
- ZW-RESIDUE-007 What is the residue of (1)/(sin z) at z=π?
- ZW-RESIDUE-008 What is the residue of (1)/(z^3(z-2)) at z=0?
- ZW-RESIDUE-009 Compute oint_|z|=1(e^z)/(z^3) dz, with the circle traversed once counterclockwise.