TITLE: States and positive elements in $C^*$-algebras
QUESTION [6 upvotes]: Let $A$ be a unital $C^*$-algebra and $w$ be a state (i.e a positive linear functional such that $\|w\|=w(1_A)=1$. I'm trying to prove the following:a) if $a$ is selfadjoint and $w(a^2)=w(a)^2$ then for any $b\in A$ we have $w(ab)=w(a)w(b)=w(ba)$.b) if $\|a\|\le 1$ and  $T=\left( \begin{array}{cc}
1_A & a  \\
a^* & 1_A \\
\end{array} \right) $ then $T$ is positive.
What I have done: 
for part a) I proved that for any $a\in A$ we have $|w(a)|\le w(|a|^2)^\frac{1}{2}$ and I know that for any state we can define a semi inner product as follows $<a,b>=w(b^*a)$ but I cant get the result I want. 
for part b) it is clear that $T$ is selft adjoint, so if $\sigma(T)\subset[0,\infty)$ then $T$ is positive, I'm not sure how to find the spectrum of $T$. 
Thank you for your help.

REPLY [5 votes]: For a): as you recalled, we have Cauchy-Schwarz for $w(x^*y)$, i.e.
$$
|w(x^*y)|^2\leq w(x^*x)w(y^*y)\quad\forall x,y\in A.
$$
Now for every $b$ in $A$
$$
w(ab)-w(a)w(b)=w((a-w(a)1)b)=w(a'b)\quad\mbox{with}\quad a'=a-w(a)1.
$$
Note that $a'$ is self-adjoint and $a'^*a'=a'^2=a^2-2w(a)a+w(a)^21$. Hence $w(a'^2)=w(a^2)-w(a)^2=0$ by assumption. Therefore, by Cauchy-Schwarz:
$$
|w(a'b)|^2\leq w(a'^*a')w(b^*b)= w(a'^2)w(b^*b)=0\quad\Rightarrow\quad w(a'b)=w(ab)-w(a)w(b)=0$$
for every $b\in A$. It follows that $w(ab)=w(a)w(b)$, and we can show in a similar fashion that $w(ba)=w(b)w(a)$.
For b): observe the matrix identity
$$
(I-T)^*(1-T)=\left(\matrix{0&-a\\-a^*&0}\right)\left(\matrix{0&-a\\-a^*&0}\right)=\left(\matrix{aa^*&0\\0&a^*a}\right).
$$
It follows that $\|I-T\|^2=\|(I-T)^*(I-T)\|=\max\{\|aa^*\|,\|a^*a\|\}=\|a\|^2$.
If $\|a\|\leq 1$, we get $\|I-T\|\leq 1$, so $T=I-(I-T)$ is positive.