# Roots of a polynomial equation

I have the following polynomial equation of 3th degree :

$$-\chi h\beta x^{3}+\chi h\beta x^{2}-2x+\left(1-\rho\right)=0$$

where $\chi$, $h$, $\beta$ and $\rho<1$ are constant parameters. I am trying to figure out the number of positive, negative and complex roots for this equation. For this, I use Descartes's rule. When I look for positive roots, there are 3 times of sign change. When I put $-x$ in the polynomial to see negative roots, there is no change of sign. If I am correct, this means that there is no negative real roots. By these information, I see that there are 3 or 1 positive roots. The case where there is 1 positive root, I suppose that there are two complex roots.

I wonder if it is possible to have a condition on constant parameters to know exactly in which case I have 3 positive real roots or 1 positive (2 complex roots) ?

• are $\chi, h, \beta$ positive? Are they also below $1 \; ?$ – Will Jagy Mar 15 '18 at 22:04
• @Jagy yes, they are all positive constant parameters. They are not necessarily below 1. – optimal control Mar 15 '18 at 22:30

with positive $A = \chi h \beta$ and $B = 1 - \rho,$ take $t = -x$ and you have $$A t^3 + A t^2 + 2 t + B = 0.$$ There are no solutions with $t> 0.$ The first derivative is $$3A t^2 + 2 A t + 2 = \frac{1}{3A} \left((3At + A)^2 \; \; + \; \; A(6-A) \right)$$ so that when $A \leq 6$ the $t$ derivative is always at least $0$ and there is just a single real root with two complex roots.
If $A> 6$ you need to work more.