In the paper "The Kadison-Singer Problem" by Marcin Bownik (<https://arxiv.org/pdf/1702.04578.pdf>), the following Lemma (3.8) is proven:


**Lemma:**
Let $p, q\in \mathbb{R}[x]$ be stable monic polynomials of the same degree. Suppose that
every convex combination $(1 − t)p + tq, 0 \leq t \leq 1$, is also stable. Then for any $0 \leq t\_0 \leq 1$,
$\operatorname{maxroot}(((1 − t\_0)p + t\_0q)$ lies between $\operatorname{maxroot}(p)$ and $\operatorname{maxroot}(q)$.


**Proof:** WLOG, we can assume $0<t\_0<1$ and $\operatorname{maxroot}(p)\leq\operatorname{maxroot}(q).$
The goal is then to show that


$m\_p := \operatorname{maxroot}(p) \leq \operatorname{maxroot}(((1 − t\_0)p + t\_0q) \leq maxroot(q) =: m\_q.$ The second inequality is easy. The first is proven by contradiction:


Suppose that $(1 − t\_0)p + t\_0q$ has no
roots in $[m\_p, m\_q]$. This implies that $(1−t\_0)p+t\_0q > 0$ for all $x \geq m\_p$. Then $q(m\_p)>0$ and $q$ must have (counting multiplicity) at least 2 roots to the right of $m\_p$. Letting now $D$ be the open disk in $\mathbb{C}$ centered at $\frac{m\_p+m\_q}{2}$ and radius $\frac{m\_q-m\_p}{2}$. It is claimed then that


$$ ((1 − t)p + tq)(z) \neq 0 \text{ for all } z \in\partial D \text{ and } t\_0 \leq t \leq 1. $$


As by assumption, the convex combination of the polynomials is also real-stable, it is sufficient to check this for $z=m\_p$ and $z=m\_q$. As $p(m\_p)=0<q(m\_p)$, this is clear for $m\_p$ as $t\_0>0$.
However, for $t=1$, we have that


$$ ((1 − t)p + tq)(z)\mid\_{t=1,z=m\_q} = q(m\_q)=0. $$


Clearly contradicting the statement that is made in the paper that


$$ \inf\_{(z,t)\in \partial D\times [t\_0,1]} \vert ((1 − t)p + tq)(z)\vert>0,$$ which then itself is used to obtain a contradiction to the assumption that there are no zeros in $[m\_p, m\_q]$ via Rouche's Theorem.


So I neither see how they can claim that the infimum is positive nor how the proof should be adapted to correct for this perceived mistake. Thanks for any help.



Max-root inequality for convex combination of real-stable monic polynomials (Kadison-Singer Problem)
Indeed, the proof in the linked paper does not seem valid.


Here is a "real" proof of the lemma in question, without using complex analysis:


Let $m:=$maxroot and $q\_t:=(1-t)p+tq$, so that $q\_0=p$ and $q\_1=q$.
As in your post, without loss of generality (wlog) $m(p)\le m(q)$. If $m(p)=m(q)$, then $m(p)$ is a common root of both $p$ and $q$, and hence $m(p)$ is a root of $q\_t$ for all $t$, so that $m(q\_t)\ge m(p)$, as desired. So, wlog $m(p)<m(q)$, and then, by shifting and rescalling, wlog
\begin{equation\*}
 m(p)=0,\quad m(q)=1. 
\end{equation\*}
We want to show that
\begin{equation\*}
 m(q\_t)\ge0
\end{equation\*}
for all $t\in[0,1]$.
Let
\begin{equation\*}
 t\_\*:=\min\{t\in[0,1]\colon m(q\_s)\ge0\ \forall s\in[t,1]\}. 
\end{equation\*}
This $\min$ exists in view of the continuity of $q\_s$ in $s$ and the compactness of $[0,1]$.


Moreover, for each $t\in[0,t\_\*]$, $q\_t$ is a convex combination of $p=q\_0$ and $q\_{t\_\*}$. So, wlog $t\_\*=1$, so that
\begin{equation\*}
 m(q\_{t\_n})<0 \tag{1}
\end{equation\*}
for some sequence $(t\_n)$ such that $t\_n\uparrow1$.


One of the following two cases must occur:


*Case 1: there is a root $r\in(0,1)$ of $q$ of an odd multiplicity.* Then $q\_1=q$ changes its sign at $r$. So, for all $t\in(0,1)$ close enough to $1$, $q\_t$ will change its sign in some small enough neighborhood of $r$ and thus
will have a root close enough to $r$, which will contradict (1).


*Case 2: there is no root $r\in(0,1)$ of $q$ of an odd multiplicity.* Then $q$ does not change its sign on $(0,1)$ from $+$ to $-$ or vice versa. Therefore and because $q(0)>0$ (as was shown in your post), we see that $q\ge0$ on $[0,1]$ (and $q>0$ on $(1,\infty)$). Moreover, $1$ is a root of $q$.
Also, $q\_t>0$ on $[0,\infty)$ for all $t>0$, because $p>0$ on $(0,\infty)$. So, switching from $q$ to
$q\_t$, we will lose the (multiple) root $1$ of $q$, as well as all the other roots of $q$ in $(0,\infty)$ (all of them of an even multiplicity). Moreover, if $t$ is close enough to $1$, this loss cannot be regained elsewhere. Thus, we get a contradiction with the assumption that $q\_t$ is stable (that is, with the assumption that all roots of $q\_t$ are real).




---


**Details on the loss and no gains in Case 2:** We have $q\_t/t=q+sp$, where $s:=(1-t)/t$ and $t:=t\_n$. So, $t\uparrow1$ and hence $s\downarrow0$. For a real polynomial $P$ and an interval $I\subseteq\mathbb R$, let $N\_I(P)$ denote the number of roots of $P$ in $I$ (counting the multiplicities), so that $N\_{\mathbb R}(P)$ is the number of real roots of $P$; moreover, $N\_{\mathbb R}(P)$ equals the degree of $P$ if $P$ is stable.


In view of (1), $N\_{(0,\infty)}(q+sp)=0$, whereas $N\_{(0,\infty)}(q)\ge2$ (this is what was referred to as the loss). On the other hand, the condition that the $q\_t$'s are stable monic polynomials of the same degree implies that $N\_{\mathbb R}(q+sp)=N\_{\mathbb R}(q)$. So, to get a contradiction, it suffices to show that
\begin{equation\*}
 N\_{(-\infty,0]}(q+sp)\le N\_{(-\infty,0]}(q) \tag{2}
\end{equation\*}
for all small enough $s>0$ (this is what was referred to as the no-gain).


By standard and easy bounds on the roots of a polynomials and by compactness, eventually (that is, for all small enough $s>0$) every root of $q+sp$ will be in arbitrarily small neighborhood of a root of $q$. Let $v$ be any (necessarily real) root of $q$, so that
\begin{equation\*}
 q(x)=(x-v)^l q\_1(x)
\end{equation\*}
for all real $x$, where $l$ is the natural number equal the multiplicity of the root $v$ of $q$, so that $q\_1$ is a polynomial with $b:=q\_1(v)\ne0$.
Let then
\begin{equation\*}
 p(x)=(x-v)^k p\_1(x)
\end{equation\*}
for all real $x$, where $k$ is the unique nonnegative integer such that $p\_1$ is a polynomial with $a:=p\_1(v)\ne0$.


One of the following two cases must occur:


*Case 2.1: $l\le k$.* Then, with $u:=x-v$,
\begin{equation\*}
 q(x)+sp(x)=u^l(q\_1(x)+su^{k-l}p\_1(x))\sim bu^l
\end{equation\*}
as $s\to0$ and $x\to v$, so that $N\_V(q+sp)=l=N\_V(q)$ for some neighborhood $V$ of $v$ and all small enough $s>0$.


*Case 2.2: $l>k$.* Then, again with $u:=x-v$,
\begin{equation\*}
 q(x)+sp(x)=u^kp\_1(x)\big(R(u)u^{l-k}+s\big),
\end{equation\*}
where
\begin{equation\*}
 R(u):=\frac{q\_1(v+u)}{p\_1(v+u)}. 
\end{equation\*}
Note that $(R(u)u^{l-k})'=R(u)(l-k)u^{l-k-1}+R'(u)u^{l-k}\sim\dfrac ba\,(l-k)u^{l-k-1}$ as $u\to0$. So, $R(u)u^{l-k}$ is monotonic with nonzero derivative in $u$ in a right neighborhood of $0$ and in a left neighborhood of $0$. It follows that the equation $R(u)u^{l-k}+s=0$ has no more than two roots $u$ in a neighborhood of $0$ if $l-k$ is even (and hence $\ge2$, by the Case 2.2 condition) and no more than one root $u$ in a neighborhood of $0$ if $l-k$ is odd. So,
$N\_V(q+sp)\le k+2\le l=N\_V(q)$ for some neighborhood $V$ of $v$ and all small enough $s>0$ if $l-k$ is even, and $N\_V(q+sp)\le k+1\le l=N\_V(q)$ for some neighborhood $V$ of $v$ and all small enough $s>0$ if $l-k$ is odd.


So, in all cases $N\_V(q+sp)\le N\_V(q)$ for some neighborhood $V$ of $v$ and all small enough $s>0$. Thus, (2) is proved. $\Box$


