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What does zero at infinity mean?


Well-known member
Feb 1, 2012
For the following algebraic expressions for the Laplace transform of a signal, determine the number of zeros located in the finite \(s-\)plane and the number of zeros located at infinity.

Does that mean pole since at a pole the plot will diverge to \(\infty\)? Otherwise, what would be a zero at infinity?


Well-known member
Jan 26, 2012
Consider the complex-valued function of a complex variable, $f(z) = z^{-2}$. We notice that as $|z|\to \infty$ that $f(|z|) \to 0$ so we say $f(z)$ is zero at infinity. I think when someone asks for "number of zeros at infinity" they are asking for the multiplicity of $\infty$ as a zero.

In general, if $f(z)$ sends $\infty$ to zero then,
$$ \lim_{z\to 0} f\left( \frac{1}{z} \right) = 0 $$
We can define the function,
$$ g(z) = \left\{ \begin{array}{ccc} f(z^{-1}) & \text{ if }& z\not = 0 \\ 0 & \text{ if }& z = 0 \end{array} \right. $$
Now count the multiplicity of $0$ for $g(z)$ at $0$.

In the example given above $f(z)=z^{-2}$ then $g(z) = z^2$ so $g(z)$ has multiplicity two zero. Thus, $f(z)$ has two zeros at infinity.