Descartes' rule of signs, Rolle's theorem and sequences of admissible pairs (1805.04261v1)

Abstract: Given a real univariate degree $d$ polynomial $P$, the numbers $pos_k$ and $neg_k$ of positive and negative roots of $P^{(k)}$, $k=0$, $\ldots$, $d-1$, must be admissible, i.e. they must satisfy certain inequalities resulting from Rolle's theorem and from Descartes' rule of signs. For $1\leq d\leq 5$, we give the answer to the question for which admissible $d$-tuples of pairs $(pos_k$, $neg_k)$ there exist polynomials $P$ with all nonvanishing coefficients such that for $k=0$, $\ldots$, $d-1$, $P^{(k)}$ has exactly $pos_k$ positive and $neg_k$ negative roots all of which are simple.