Cauchy index

In mathematical analysis, the Cauchy index is an integer associated to a real rational function over an interval. By the Routh–Hurwitz theorem, we have the following interpretation: the Cauchy index of

over the real line is the difference between the number of roots of f(z) located in the right half-plane and those located in the left half-plane. The complex polynomial f(z) is such that

We must also assume that p has degree less than the degree of q.

We recognize in p(x) and q(x) respectively the Chebyshev polynomials of degree 3 and 5. Therefore r(x) has poles ${\displaystyle x_{1}=0.9511}$, ${\displaystyle x_{2}=0.5878}$, ${\displaystyle x_{3}=0}$, ${\displaystyle x_{4}=-0.5878}$ and ${\displaystyle x_{5}=-0.9511}$, i.e. ${\displaystyle x_{j}=\cos((2i-1)\pi /2n)}$ for ${\displaystyle j=1,...,5}$. We can see on the picture that ${\displaystyle I_{x_{1}}r=I_{x_{2}}r=1}$ and ${\displaystyle I_{x_{4}}r=I_{x_{5}}r=-1}$. For the pole in zero, we have ${\displaystyle I_{x_{3}}r=0}$ since the left and right limits are equal (which is because p(x) also has a root in zero). We conclude that ${\displaystyle I_{-1}^{1}r=0=I_{-\infty }^{+\infty }r}$ since q(x) has only five roots, all in [−1,1]. We cannot use here the Routh–Hurwitz theorem as each complex polynomial with f(iy) = q(y) + ip(y) has a zero on the imaginary line (namely at the origin).

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